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Structural geology 

Charles Kenneth Leith 



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STRUCTURAL GEOLOGY 



BY 
C. K. LOTH 

CSTVTKS/TT OT WKO«N>iX 




NEW YORK 

HENRY HOLT AND COMPANY 

1913 



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IpUdLICLIoRARY 

730708 

ASTO!\ ' ' '«J'.''\ A.\D 

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Copyright, 1913 



*HEtf#Y feOL^'AttD tfcfs&ANY 



PRESS OF T. MOKEY & SON, 
GREENFIELD, MASS , U S A. 



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INTRODUCTION 

The central feature of structural geology is the interpretation of 
structures produced in rocks by earth movements. The outer 
limits of structural geology are not clearly defined, for in one way 
or another the subject is interrelated with nearly all phaw« of 
geology. Its purpose is the study and interpretation of rock 
structures, not for themselves, but for the light they may throw on 
stratigraphic problems, on economic geology, on the causes under- 
lying the general configuration of the earth, and on earth's history. 

The structural geologist has in recent years found it necessary 
in his field work to give mtjch '.attention to the#enotic relationships 
of rock structures produced by deformation. . Some of these rela- 
tionships have not yet found eVpressioit in the available literature 
on the siiUftect. The student Veals' jn gent/^f text-lxjoks about 
individual structures but seldom;^ their relations, with the result 
that at least in his early field work he may fail to utilize methods 
which are helpful or essential in the interpretation of the geology of 
a district. Emphasis upon geological structures as related parts 
of a record or process rather than as isolated facts determines 
the method of presentation in this book. Illustrations are chosen 
principally from the United States. 

Primary structures of rocks, such as bedding and igneous st ma- 
tures, are to be considered in the study of structural geology, but 
these receive more or less adequate treatment in stratigraphic and 
petrographic geology. The writer will therefore treat these sub- 
jects only incidentally, putting the emphasis on secondary struc- 
tures developed in rocks by earth movements. 

The writer is indebted to Professor Eliot Blackwelder of the 
University of Wisconsin for several of the illustrative examples of 
the expression of structures on the erosion surface, and to Pro- 
fessor W. J. Mead for important suggestions relating to experi- 
mental deformation. Greatest of all is the writer's obligation to 
President C. R. Van Hise, who as teacher and associate in geologi- 



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iv INTRODUCTION 

cal field work originated and developed many of the ideas expressed 
in this book. For some years the structural discussion in Van 
Hise's " Principles of North American Pre-Cambrian Geology" 1 
has been widely used by American teachers of structural geology. 
The writer had the privilege of association with Dr. Van Hise in the 
development of that work, and the present volume is partly a 
development and revision of the ideas of that paper. 

1 Van Hise, C. R., Principles of North American Pre-Cambrian Geology: 16th 
Ann. Rept. U. S. Geol. Survey, pt. 1, 1896, pp. 571-874. 



-•• • 
• • • 



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CONTENTS 

PAGE 

INTRODUCTION iii 

FRACTURE AND FLOW 1 

Kinds of fracture and flow structures .... 1 

Distribution of fracture and flow structures ... 1 

Conditions favoring fracture or flow 4 

' J Depth necessary for rock flow 9 

Volume changes in fracture and flow . . . .11 

Surface expression of the zones of fracture and flow . 12 

FRACTURES 14 

Attitudes of fractures with reference to stresses . . 14 

Tension fractures 14 

Compression fractures 16 

Joints 21 

Joints which can be classified as due to tension ... 22 

Joints which can be classified as due to compression . . 23 

Joints developed under unknown stress-strain conditions . 28 

Widening of joints by the linear force of growing crystals . 29 

Surface expression of joints 30 

Suggestions for laboratory work on joints .... 31 

FAULTS 31 

Nomenclature 32 

Apparent and real fault displacements 36 

Normal faults 39 

Normal faults associated with igneous rocks ... 42 
Normal faults in unfolded sediments .... 43 
Association of normal faults with folds .... 43 
Vertical and steeply-dipping normal faults and joints in in- 
tersecting systems 44 

Reverse or thrust faults 46 

Distributive thrust faults 48 

Faults with horizontal displacements 50 

Hinge or pivotal faults . 50 

Curved and folded faults 51 

Faults passing into folds or into schistose zones . .51 

Correlation of faults 53 

Relative number of normal and reverse faults ... 54 
Relative shortening and elongation of the earth's crust by 

faulting 9 55 

v 



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vi CONTENTS 

FAULTS— Continued page 

Evidence of faulting 56 

Surface expression of faults 57 

Suggestions for laboratory study of faults .... 60 

Fracture cleavage and fissility 61 

Breccias and autoplastics 64 

Earthquakes 67 

Earthquakes as cause and effect of rock fracture . . . 67 

Kinds of fracturing accompanying earthquakes ... 69 

Earthquakes and oscillations of glaciers 70 

Earthquakes and vulcanism 70 

Earthquakes and magnetic disturbances .... 70 

Earthquakes and rock density 70 

Earthquake zones 71 

Instruments for determining and measuring earthquakes 71 

Earthquake waves 72 

Condition of earth's interior as inferred from earthquake 

waves 72 

Location of the origin of earthquakes 73 

Prediction of earthquakes 74 

ROCK FLOWAGE 76 

Flow cleavage 76 

Manner in which the parallel arrangement of minerals is brought 

about 79 

Recrystallization 79 

Granulation and rotation of original particles ... 82 

Cleavage in its relations to differential pressures ... 84 

Relations of cleavage to strain 85 

Relations of cleavage to stress ...... 86 

Gneissic structure 87 

Idiomorphic or porphyritic textures developed by rock flow- 
age 90 

Rock flow age without retention of cleavage ... 92 

Obliteration of textures by rock flowage .... 93 

Identification of schists and gneisses 97 

Field relation as a means of identifying schists and gneisses 97 
Mineral composition as a means of identifying schists and 

gneisses 98 

Chemical composition as a means of identifying igneous or sed- 
imentary origin of gneisses and schists .... 100 
Conclusion as to methods of identifying gneisses and schists 102 ' 

STRUCTURES COMMON TO BOTH FRACTURE AND FLOW . 104 

Folds 104 

Elements of folds 104 

Folds in the zone of fracture and zone of flow contrasted . 108 



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CONTENTS vii 

STRUCTURES COMMON TO BOTH FRACTURE AND FLOW— 

Continued page 
Control of structures in weak beds by differential movements 

between competent beds on limbs of folds . .114 

(1) Minor folds as evidence of differential movement be- 

tween beds 114 

(2) Cleavage as evidence of differential movement in folding 119 

(3) Jointing, fracture cleavage, and fissUity as evidences of 
differential movement between beds in folding . .121 

Localization of folds 124 

Determination of depth affected by folds .... 124 

Field observations on folds 127 

Strike and dip 127 

Emphasis on relations of major and minor structures . . 128 

Field observations on relations of cleavage to folds . . 128 
Determination of top and bottom of sedimentary beds in a 

folded area 132 

Suggestions for laboratory study of folds .... 134 

MOUNTAINS 136 

Types of mountains 136 

Mountains and normal faults 137 

Mountains and thrust faults 137 

Mountains and folds 137 

More complex relations of mountains to structure . . 138 

Localization of mountains 138 

Suggestions for laboratory study of mountains . 140 

MAJOR UNITS OF STRUCTURE (GEANTICLINES, GEOSYN- 
CLINES, OCEAN BASINS, CONTINENTS, PLATEAUS, POSI- 
TIVE AND NEGATIVE ELEMENTS) 141 

Shapes of major elements of structure 141 

Actual and apparent uplifts 143 

ULTIMATE FORCES OF SECONDARY DEFORMATION . 144 

Outline of principal theories 144 

Isostasy 145 

Support of hypothesis by recognition of weakness of rocks 145 

Dutton's and Gilbert's observations on isostasy . . . 146 

Hayford's observations on isostasy 146 

Earth movements in relation to isostasy 148 

Isostasy in relation to rigidity of rocks 148 

Depth of isostatic compensation 149 

Criticism of theory of isostasy 149 

Causes of tension 152 

Conclusion as to major causes of deformation . . . 152 

Local and minor causes of deformation 153 

Relation between deformation and vulcanism . . .154 



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viii CONTENTS 

PAGE 

UNCONFORMITY 156- 

Identification of unconformity 157 

Interpretation of unconformity 159 

Suggestions for laboratory study of unconformity . , . 161 ' 

INDEX 163 



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STRUCTURAL GEOLOGY 



FRACTURE AND FLOW 

KINDS OF FRACTURE AND FLOW STRUCTURES 

Rocks are deformed by fracture and by flowage. Rock fractures 
are known geologically as joints, faults, brecciation, autoplastic 
structures, fracture cleavage, etc. 

Rock flowage may be defined as a permanent change of form by 
pressure without conspicuous fracture. It does not include igneous 
fusion. It is accomplished by interior readjustments of rock 
substances by chemical, mineralogical, and mechanical changes, 
these changes being favored by high pressure and temperature, 
moisture, and by the presence of rock substance easily susceptible 
to these changes. The results of rock flowage are commonly a 
parallel arrangement of the constituents of the rock mass, produc- 
ing a schistosity, cleavage, or banded structure. Where the rock 
is made up of minerals not adapted dimensionally to taking on a 
parallel arrangement, rock flowage may leave no evidence of itself 
in parallel arrangement. There are gradational structures between 
flow and fracture, for rock may be deformed mainly by minute 
fracture or slicing and still be a coherent mass. It has, in effect 
flowed. 

Folds are developed by both flowage and fracture. 

DISTRIBUTION OF FRACTURE AND FLOW 
STRUCTURES 

The prevailing manner of deformation at the earth's surface is 
by fracture, as is known by observation and experiment. 

The prevailing manner of deformation deep below the surface 
may be inferred to be by flowage. Rock flowage has been actually 
observed in process, as, for instance, the flowage of schists in the 
Simplon and other deep tunnels and the creep of soft shales in 

1 



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2 STRUCTURAL GEOLOGY 

mines. For the most part, however, rock flowage takes place at 
depths beyond our range of observation, and our conclusions as to 
the existence, locus, and conditions of a zone of rock flowage rest 
principally on inference. We observe some rocks at the earth's 
surface with textures which have been developed by rock flowage. 
We see that flowage is not now taking place in them. We know 
that the rocks were once far below the surface and now appear at 
the surface because erosion has uncovered them. We conclude 
that they flowed when beneath the surface, under physical condi- 
tions other than those under which they now rest, and that there- 
fore rocks are today flowing beneath the surface. We reverse the 
statement of the Huttonian principle that the present is the key 
to the past, and argue that the past is the key to the present. 

Artificial rock flowage may be accomplished under conditions 
which seem to us probably analogous to those existing at depth. 
(See p. 4.) 

The existence of a zone of rock flowage beneath the surface is 
inferred also from the behavior of earthquake waves. These are 
initiated by the shock of fracturing; and it is significant that their 
point of origin, as determined by many independent observers, has 
never been found to be far below the surface. This fact indicates 
that fractures go only to a comparatively shallow depth, and that 
below this rock deformation must be accomplished in some other 
way. 

The wrinkling of the earth's surface into mountain ranges in- 
volves a slipping of the crust which renders plausible the existence 
of a zone of flow. 

If the earth's surface is in a state of isostatic adjustment, the 
conclusion seems inevitable that this adjustment has been main- 
tained by means of deep-seated flowage to compensate for trans- 
ference of surface loads by erosion. 

It is concluded, therefore, partly by direct observation but 
largely by inference, that somewhere beneath the earth's crust is a 
zone in which deformation is by flowage. It may seem super- 
fluous to use so many words to argue that there is a zone of rock 
flowage yet if we think to ask ourselves how we know this, we are 
obliged to confess that inference has been an important factor 
in reaching this conclusion. Actual observation does not go be- 
low a zone of combined fracture and flow. Even the Keewatin 



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DISTRIBUTION OF STRUCTURES 3 

and Laurentian rocks, the oldest of the pre-Cambrian of North 
America, have only partly undergone rock flowage, and even in 
these rocks the flowage is in considerable part a direct result of 
plutonic intrusion rather than depth alone. 

The existence of a zone of fracture and a zone of flow was in- 
ferred by Heim 1 from his studies of the Alps. Gilbert 2 also sep- 
arated the two types of deformation on basis of depth, but did not 
use the term zone. Van Hise 3 first proposed a classification of the 
lithosphere on a basis of vertical distribution of the dominant 
kinds of deformation, into an upper zone of fracture, a middle 
zone of combined fracture and flowage, and a lower zone of flowage. 
In view of the fact that flowage in certain soft rocks may begin 
almost at the surface, nearly all of the zone of the lithosphere 
within our range of observation is that of combined fracture and 
flowage. Also rocks which have been deformed by flowage below 
the surface in the past and are now exposed by erosion lie along- 
side of rocks now being fractured at the surface within our range of 
observation. The depth necessary for flowage differs for different 
rocks, and is dependent upon a variety of conditions. A general 
statement of the distribution of structures is that at the surface 
most rocks fracture and some flow; that far enough below the sur- 
face all rocks may flow. Van Hise emphasized the variation of 
depth of the zones of fracture and flow for different rocks and under 
different conditions; but the use of the word "zone" has caused un- 
due stress to be placed on uniformity of depth by students who 
have used these terms. The emphasis should rather be on condi- 
tions. As expressed by a student in an examination, the zone of 
fracture or flQwage "like Heaven, is a condition, not a place." 
If "zone " were understood to convey the notion of both condition 
and place, it would more clearly express the fact. A hard quartzite 
fractures, while a shale lying either above or below may flow. A 
quartzite may fracture at one place, while near at hand, without 
increase of depth but under different conditions, it may flow. In 
order that the terms "zone of fracture" and "zone of flow" may 
have definite significance, they should be related to specific rocks, 

1 Heim, Albert, Untersuchungen uber den Mechanismus der Gebirgsbildung, 
Basel, 1878. 

2 Gilbert, G. K., Geology of the Henry Mountains: 2nd ed., Washington, 1880. 

3 Van Hise, C. R., Principles of North American Pre-Cambrian Geology: 16th 
Ann. Kept. U. S. Geol. Survey, p. 589. 



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4 STRUCTURAL GEOLOGY 

for instance, "the zone of fracture for quartzite," "the zone of flow 
for shale." 

As rocks approach the earth's surface by erosion of overlying 
rocks or through volcanic agencies they become fractured and 
disintegrated. As they are buried beneath the surface they may 
come under conditions of rock flowage which weld and integrate 
them. Structural changes may thus be in cycles. As the depths 
of fracture and flow vary widely for different rocks and under 
different conditions, one rock may be in the destructive phase of its 
structural cycle while a nearby rock may be in a constructive phase. 
The terms "zone of fracture" or "zone of flow" may therefore be 
considered as applying to a given rock in a phase of its structural 
cycle. Depth is only one of the important factors determining the 
phase of the cycle. 

CONDITIONS FAVORING FRACTURE OR FLOW 

Most rocks fracture at the surface; some of them flow. It may 
be supposed that far enough below the surface all of them may 
flow. Practically, our zone of observation is that of combined 
fracture and flow. These kinds of deformation may occur side by 
side in different rocks or in the same rocks. The specific combina- 
tion of factors which determines fracture rather than flow in the 
given location can seldom be more than approximately ascertained. 

Rock flowage has been experimentally accomplished on a small 
scale. Kick * in 1892 put crystals in a copper box, filled the space 
with imbedding material such as paraffine wax and fusible metal, 
covered the box with brass plates, and put it under great pressure. 
The resistance to deformation offered by the copper as well as by 
the imbedding material is transmitted through the bedding ma- 
terial to the specimen, which thus receives a very considerable 
lateral support. In this manner Kick secured permanent deforma- 
tion in salt, talc, gypsum, fluorspar, and marble. 

Adams subsequently repeated these experiments on a more 
elaborate scale, using a variety of limestones and marbles, with 
similar results. 2 This method produces rock flowage. The essen- 

1 Kick, Prof. Friedrich, Die Prinzipien der mechanischen Technologie und die 
Festigkeitslehre: Zeit. des Ver. Deut. Ingen., Vol. 36, 1892, p. 919. 

2 Adams, F. D. f An experimental investigation into the action of differential 
pressure on certain minerals and rocks, employing the process suggested by Pro- 
fessor Kick: Jour, of Geol., Vol. 18, No. 6, 1910, pp. 489-525. 



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CONDITIONS OF FRACTURE OR FLOW 5 

tial condition was apparently the lateral support. The method, 
however, is qualitative, in that it is difficult to measure the pres- 
sure acting upon the specimen itself, as distinguished from that 
on the copper box and on the paraffine. 

A more nearly quantitative method, and one allowing far greater 
pressures, has been used by Adams, 1 who fitted cylinders of marble, 
granite, and diabase into steel jackets and compressed them by a 
piston to such a degree that the sides of the steel casing were made 
to bulge (see Fig. 1). All of the stresses were above the crushing 
strength of the rocks, but they differed much in intensity. When 
the casing had been cut away the rock was found to have nearly 
as great strength as it had before deformation. Similar results 
have been observed in concrete cylinders incased in steel jackets 
which have been hardened for sixteen hours and then allowed to 
stand under great pressures. The result was deformation by 
"flow." 2 

Strength tests on building stone cubes afford good illustrations 
of rock fracture. The block is compressed in one direction, the 
sides being left free. The maximum pressure required for fractur- 
ing the strongest rocks is from 25,000 to 30,000 pounds per square 
inch. 

In the most general terms, experimental results seem to show 
that when a rock is free to escape in some direction, it will break 
when under pressure greater than its crushing strength. When 
not free to escape except by exerting a pressure greater than its 
crushing strength, it flows if sufficient pressure is brought to bear 
upon it. Expressed more technically, the pressure acting upon 
any one unit of the rock mass may be resolved into three mutually 
perpendicular components, called the three principal axes of stress. 
Where one or two of these axes of stress are less than the crushing 
strength of the rock and the others are above it, the rock breaks, 
in directions determined by the relative intensities of the three 
principal stresses. Where all of the stresses are greater than the 
crushing strength of the rock, that is, when the rock mass is con- 
fined on all sides by pressures greater than its crushing strength, 

1 Adams, Frank D. t and Nicolson, J. T., An experimental investigation into the 
flow of marble: Phil. Trans. Roy. Soc. of London, Vol. 195, 1901, pp. 363-401. See 
also, Adams, Frank D., and Coker, Ernest G., The flow of marble: Amer. Jour. 
Sci., Vol. 29, 1910, pp. 465-487. 

2 Engineering News, Vol. 54, Nov. 2, 1905, p. 459. 



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STRUCTURAL GEOLOGY 




Fig. 1. Flowage of marble. After Adams, a. Columns of marble before and after 
deformation, b. Deformed column of marble as it appears in the steel jacket. 

one or more of the stresses greatly preponderating over the others, 
the rock yields by rock flowage. 

In rock flowage the stress-difference (i. e., difference in intensity 
of greatest and least of the principal stresses) necessary to deform 



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CONDITIONS OF FRACTURE OR FLOW 7 

the rock may be much greater than the crushing strength of the 
rock. Experimental evidence points in this direction, although 
there are insufficient data to warrant satisfactory quantitative 
statements. Hallock 1 has shown that a substance like a dime or 
a brass tack, when imbedded in steel and then subjected to enor- 
mous pressure, acquires a rigidity which allows deformation only 
when the stress difference has become very large. The silver 
coin acquires so great a rigidity that it will impress itself in the 
steel before flowing. Adams 2 and Pfaff 3 also found in their experi- 
ments that when rocks were under pressure enormously greater 
than their ordinary crushing strength, they would not flow through 
a small hole bored in the side of the steel jacket nor would small 
holes in the rock become closed; and it was concluded that a high 
degree of artificial rigidity had been induced in the rock, which 
could be overcome only by excessive stress difference. High 
rigidity would seem to be a probable condition deep in the earth, 
and hence enormous stress difference might be required to effect 
deformation. 

While under certain conditions of compression the rock may 
flow, it may fracture under tension stresses of equal or greater 
magnitude. The breaking strength of rocks under tension is less 
than its resistance to fracture by compression or to flowage by 
compression. 

A substance may be deformed by compressive stress at the 
same time that it is being pulled in another direction by a tensional 
stress (see. pp. 16 and 25). It is entirely conceivable, if the rock is 
soft, that under these conditions the response to compression may 
be rock flowage and the response to tension may be rock fracture, 
for it is known that under tension a rock breaks under much less 
stress than under compression, and under the higher compressional 
stresses there may be rock flowage. 

The conditions of rock flowage in the earth maybe quite different 
in some cases from those experimentally determined, due to factors 
of time, moisture, and character of the rock. Given long enough 
time, even the strongest substances may become deformed without 

1 Hallock, William, The flow of solids, or liquefaction by pressure: Am. Jour. 
Soi., Vol. 34, 1887, p. 280. 

2 Adams, Frank D., An experimental contribution to the question of the depth 
of the zone of flow in the earth's crust: Jour. Geol., Vol. 20, 1912, pp. 97-118. 

8 Pfaff, F., Der Mechanismus der Gebirgsbildung, pp. 16-19. 



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8 STRUCTURAL GEOLOGY 

fracture under stresses less than their crushing strength. For 
instance, marble gravestones sag when suspended at both ends for 
many years. Structural materials are known to do the same. In 
both cases the load is less than that necessary for crushing. 

Deformation by flowage may be facilitated by high tempera- 
ture and moisture content. Such factors favor rapid chemical 
changes and recrystallization, thereby enabling flow to take place 
more easily. It is a matter of observation that rocks have under- 
gone rock flowage by means of recrystallization of the mineral 
particles, and that such recrystallization has seemed to be at a 
maximum in rocks which were once at a high temperature, as 
near intrusive igneous contacts, or had a high content of moisture, 
or both. High temperature and moisture have been found experi- 
mentally to aid recrystallization. 

Another factor which helps to determine fracture or flow under 
given conditions is the character of the rock itself — its weakness, 
and its susceptibility of recrystallization, the latter in turn depend- 
ing on mineral content, texture, degree of hydration, and other 
conditions. Thus it is that under the same pressures one rock may 
fracture and the other flow. In general, muds, shales, slates, and 
limestones flow much more readily than the harder types such as 
quartzite and igneous rocks. 

The scope of this paper does not call for any attempt to explain 
the physical and chemical basis of recrystallization, beyond calling 
attention, as has been done, to the general factors which seem to be 
effective according to field and experimental observation. Much 
remains to be done to get these factors on a quantitative basis. 
It is entirely likely that as progress is made in this regard there will 
be a considerable change in the emphasis on the several factors 
cited. For instance, the presence of moisture seems to favor 
recrystallization, judging from field conditions. Of two rocks of 
different moisture content, the one containing the more water 
seems to recrystallize more readily, yet in experiments in the 
artificial recrystallization of minerals in the Carnegie Institution of 
AVashington it has been found that recrj'stallization occurs with 
unexpected readiness under conditions of dry heat. Artificial 
rock powders when heated dry have been found to recrystallize, 
giving particles large enough for microscopic study. 



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DEPTH OF ROCK FLOWAGE 9 

DEPTH NECESSARY FOR ROCK FLOW 

A shale may be deformed by flowage near the surface, while a 
brittle quartzite may require great depth. A rock at a given 
depth may fracture in one locality, while in another locality, 
because of vulcanism, or high pressure and temperature developed 
by mechanical thrust, or because of its relations to adjacent 
strata, may be deformed by flowage. Therefore no one figure 
may be taken as the depth of the zone of rock fracture. It is 
apparent that the depth beneath the surface necessary to produce 
rock flowage is only one of a number of variable factors determin- 
ing the manner of deformation of a rock. Among these are the 
following: whether stresses are tensional or compressional, varia- 
tion of minor compressive stresses and thus of induced rigidity, 
variation in strength of the materials, variation in chemical and 
mineralogical composition, variation in moisture-content and 
temperature, duration of time, and possibly other unknown varia- 
bles. Notwithstanding our lack of quantitative measurements of 
some of these factors, it is still possible to arrive at some approxima- 
tion for the minimum depth at which all rocks will flow even when 
not favored by factors other than depth. 

An early attempt to use quantitative methods in determining 
this depth was made by Van Hise and Hoskins. 1 Their calculation 
of the depth of covering which would give a pressure sufficient 
to close a cavity gave a range of from three to seven miles. In 
making this calculation they made assumptions favorable to the 
greatest depth — for instance, that the rock was of the strongest 
known kind, that conditions of temperature and moisture were 
the least favorable to recrystallization, that lateral stress was 
absent, that the pressure is lessened by the buoying effect of under- 
ground water. One of their assumptions, however, tends to make 
the calculated depth too small, namely, that the stress difference 
necessary to close a cavity is just equal to the crushing strength 
of the rock. Experiments of Adams, Pfaff, and Hallock, cited 
above, have shown that the rock acquires a high degree of rigidity 
when compressed on all sides, and that enormously greater stress 
difference is necessary to cause deformation of any kind. How 
much greater the pressure would need to be is yet uncertain. 

1 Op. cit., pp. 589-593. 



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10 STRUCTURAL GEOLOGY 

Adams 1 has shown experimentally that a cavity will not close 

11 miles below the surface at a temperature of 550° C. even if a 
pressure is used that is 50% greater than that obtaining at this 
depth. For granite, in fact, he finds that cavities remain open 
at ordinary temperatures even with pressures corresponding to 
depth of 30 miles. These experiments may lead to overestimate of 
depth for flowage in general, for the reason that the cavities used 
were very minute, the factor of moisture was not included, and the 
time element was only partially accounted for by increasing the 
pressure. Also with larger openings than used in the experiments, 
presumably less stress difference would be required to close cavities. 
Under the conditions of the experiment cubical compression 
played an important part. 

A factor not considered in the above estimates is the fact that 
under tension of whatever magnitude the rock will fracture rather 
than flow. So far down in the earth as tension exists, therefore, 
rock fracture may extend. 

The possibility is suggested on page 7 that a rock may yield 
to compression by flowage — at the same time it is yielding to 
tension by fracture. If this is possible, the fractures are really 
minor and subsidiary to the flowage and therefore require only a 
minor modification of our discussion of the depth at which a rock 
will flow. 

Estimates of the depth of the zone of rock fracture have also 
been made by studying the amount of erosion necessary to uncover 
evidences of rock flowage. This method, by its very nature, must 
yield indefinite results; and yet, as applied in different parts of the 
world by different observers, it indicates that the depths below the 
surface necessary for rock flowage for strong rocks are possibly a 
little larger than those derived from the computations of Van Hise 
and Hoskins. 

Another line of evidence on the same point is afforded by a 
study of earthquake shocks. Earthquakes originate in the frac- 
turing of rocks, and in no case has their point of origin been esti- 
mated to be more than nine or ten miles below the surface. Also 
it has been found that waves traveling along a chord which passes 
ten or twelve miles below the surface at the deepest point are 

1 Adams, Frank D., An experimental contribution to the question of the depth 
of the zone of flow in the earth's crust: Jour. Geol., Vol. 20, 1912, p. 115. 



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DEPTH OF ROCK FLOW 11 

sharply discriminated in speed and in position of their planes of 
vibration from waves traveling along the circumference. Waves 
traveling along chords at shallower depths are not thus easily 
differentiated. Some difference in medium at great and small 
depths is assuredly indicated. 

Suggestive, but not to be cited as definite evidence, is the fact 
that the mountains of the earth's crust never rise much above five 
miles in height. There are many factors which control summit 
levels. One of them has been suggested to be the yielding of the 
rocks at the base by flowage when the mountains had reached a 
height of over five or six miles. Prevalence of flowage structures 
in the cores of mountains are in accord with this view, though many 
of them may be otherwise explained. 

From various sources, therefore, there is evidence or suggestion 
that the zone of rock fracture is comparatively shallow, perhaps 
less than twelve miles deep for the strongest rocks. No one line 
of evidence cited is decisive. Yet there is such accordance of the 
various kinds of evidence that the figures above given may be 
tentatively accepted. The figures may be increased when more is 
known of the ratio of rigidity to increase of depth. 

VOLUME CHANGES IN FRACTURE AND FLOW 

Fracturing itself involves increase of volume of the fractured 
mass, because of displacement of the parts. In the zone of fracture 
rocks also are accessible to weathering agencies of the atmosphere 
and hydrosphere and undergo metamorphic changes which increase 
their volume. Calculations of the changes of volume of the com- 
mon rocks of the earth's crust indicate a maximum increase in 
volume at the surface of 50% by development of pore space and of 
minerals of low density. In the zone of flow there is a tendency to 
diminish volume by closing pore space and by developing minerals 
of higher density. 

If the three principal stresses are equivalent, the rock may be 
cubically compressed, but experimentally no permanent compres- 
sion has been accomplished, the rock expanding as soon as pressure 
is released. The experiments of Adams 1 show that acid rocks are 

1 Adams, Frank D., and Coker, Ernest G. t An investigation into the elastic con- 
stants of rocks, more especially with reference to cubic compressibility: Pub. 
No. 46, Carnegie Inst, of Wash., 1906, pp. 66-68. 



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12 STRUCTURAL GEOLOGY 

more elastic than glass, basic rocks less so, marbles and limestones 
about the same. Of the minerals, quartz is highly elastic, and 
therefore he concludes that the high elasticity of granite is probably 
due to this cause. 



SURFACE EXPRESSION OF THE ZONES OF FRACTURE 

AND FLOW 

Erosion takes advantage of fracture planes in etching the 
earth's surface. Where rocks are homogeneous and the fracture 
planes are in well-defined systems, drainage lines may be in more 
or less regular patterns, especially in non-glaciated regions. Where 
fractures are curved and discontinuous and not in regular systems, 
this may be represented in the irregularity of the erosion channels. 
It must be remembered that fractures are not the only structures 
which localize erosion channels. Differing resistance of rocks, 
bedding, dip of impervious layers, etc., have their influence. Hence 
it should not be assumed that all drainage patterns correspond to 
fracture systems, and it is especially unsafe to read into the actual 
pattern a more regular pattern based on a hypothetical conception 
of fracture systems. 

Dislocations of the earth's crust may, independently of erosion, 
cause topographic irregularities, some of which are referred to in a 
later section on faults (pp. 57-58) . 

Rocks which have undergone rock flowage are for the most 
part easily eroded, and are consequently likely to be relatively low 
areas. Schistosity obliterates expression of original structures. 
The schistose structure resulting from rock flowage may give linear 
elements to the topography, but these elements are likely to be 
curving, overlapping, discontinuous, and not in the more or less 
regular intersecting sets characteristic of rock fracture. 

By the time erosion has exposed at the surface rocks which have 
undergone rock flowage, these have come through the zone of 
rock fracture, with the result that fractures may be superposed 
upon schistosity, in which cases the surface expression may com- 
bine features characteristic of rock flow and fracture. 

Other things being equal, evidence of flowage is likely to be 
more conspicuous in areas from which there has been a large 
amount of material eroded than elsewhere. The rocks showing at 



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SURFACE EXPRESSION OF STRUCTURES 13 

the surface in such areas have been buried to great depths Lelow 
the surface. Older rocks are likely to have been more deeply 
buried below the surface than younger rocks, and therefore to have 
been at one time in the zone of rock flowage, but this does not 
always follow. 

It is frequently possible to determine from the study of geologic 
and topographic maps, whether the rocks of an area are characteris- 
tic of the zone of rock flowage or rock fracture. Note for instance 
the contrast between Archean and Algonkian areas in most parts 
of the Lake Superior region, and between the pre-Cambrian and 
Paleozoic areas in the Piedmont and southern Appalachians. 
The student may study to advantage the evidences of fracture and 
flow on the following maps with their accompanying sections: 

Roan Mountain folio, Tennessee-North Carolina, No. 151, U. S. G. S. 
Pisgah folio, North Carolina-South Carolina, No. 147, U. S. G. S. 
Gadsden folio, Alabama, No. 35, U. S. G. S. 

Geology of the Lake Superior Region, Mon. 52, U. S. G. S., particularly 
maps of the Marquette and Gogebic districts. 

Note the areal distribution and relations of rocks, presence of 
linear elements, evidences of thickening or thinning by flowage, 
schistosity, drainage, depth to which rocks have been covered, etc. 
More specific expressions of the zones of fracture and flow at the 
rock surface are discussed on later pages. 



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FRACTURES 

Rock fractures are usually designated by terms such as joints, 
faults, fracture cleavage, autoclastic structures, etc. The variety 
of names and classifications of rock fractures should not obscure 
the fact that after all they are only expressions of the ordinary 
mechanical principles of the breaking of solid materials. We 
may for the time avoid some complexity of names, therefore, by 
outlining first some of the simpler mechanical features of the frac- 
turing of rocks, applicable to all rock fractures regardless of names. 
To do this adequately would require the use of many of the techni- 
cal terms of mechanics, which, for the purpose of this volume, 
would be undesirable. In the following account of the principles 
of fracturing the attempt is made to use non-technical language, 
even though this may seem to the technical reader to be at the 
expense of accuracy and conciseness. Some technical terms are 
unavoidable. 

ATTITUDES OF FRACTURES WITH REFERENCE TO 

STRESSES 

Stress is defined as the reaction of the interior parts of a solid 
against forces tending to deform it, and strain is the change in 
shape of the solid resulting from these reactions. All stresses act- 
ing at any point may be resolved into three mutually perpendicular 
components or principal axes of stress. There are correspondingly 
principal axes of strain. 

Fractures form in the following relations to stress: 

TENSION FRACTURES 

Under tension, fractures tend to develop in planes normal to 
the maximum stress. There are also shearing stresses inclined to 
the maximum tension, just as there are in compression (see p. 16), 
but only rarely does the breaking of the rock mass follow these 

14 



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TENSION FRACTURES 



15 



planes of shearing stress, because the resistance of the rock to 
tension is less than its resistance to shearing. 

Tension fractures may develop also when a mass is deformed by 
shearing in the manner described on page 16. 

By torsion, intersecting sets of fractures have been simulta- 
neously produced at angles of 45° to the axis of torsion. These 




Fio. 2. Diagram to illustrate the development of rectangular sets of tension frac- 
tures under torsion. After Daubree. 



fractures are probably due to tension rather than compression. If a 
circle be drawn on the flat side of a rubber eraser and the eraser 
twisted it will be noted that the elongation of this circle, indicating 
tension, is normal to the planes followed by fracture in torsion 
tests 1 (Fig. 2). 

1 Becker, G. F., The torsional theory of joints: Trans. Am. Inst. M. E., Vol. 24, 
1895, p. 136. 



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16 STRUCTURAL GEOLOGY 

COMPRESSION FRACTURES 

Under compressive stresses, fractures tend to develop along 
planes of " maximum shear," which are inclined to the direction of 
principal stresses; but the degree of inclination and the direction of 
dip of the planes away from the direction of maximum stress vary- 
between the following limiting cases: 

A building stone cube, subjected to pressure on one pair of 
opposite sides, shears in planes 45° or less from the line of maximum 
pressure. These planes may dip in one direction away from this 
line or may dip outward in all directions, developing a cone. If the 
cube be subjected to pressure as before, while it is being rigidly sup- 
ported on another pair of opposite sides, the remaining surface 
being free, fractures will develop dipping toward the free sides. 
Portions of the rock mass will thus be displaced in the direction 
of these free sides. This presumably is a common case in nature, 
as, for instance, where a horizontal stress affecting a homogeneous 
rock mass is relieved principally by displacement upward. The 
planes of fracture dip from the surface toward the greatest com- 
pression and displacement along these planes will carry the rock 
mass upward, in the manner of a thrust fault. 

The compressive strains thus far described are known as non- 
rotational) l that is, the principal directions of stress remain constant 
with reference to the principal axes of strain throughout the defor- 
mation. Fully as common in nature are rotational strains or 
shears, in which the strain axes are being constantly rotated during 
the deformation, illustrated by Fig. 7. The fractures are then not 
symmetrically grouped with reference to the principal stress but 
they retain much the same relations to the elongation and short- 
ening of the deformed mass, as in the case of non-rotational strain 
above described. The principal stress usually intersects the obtuse 
angle between such fractures. 

One of the incidental accompaniments of fracture by shearing 
under a rotational compressional stress may be development of 
tension fractures in planes normal to the elongation of the mass. 

A convenient way to remember and picture the system of frac- 
tures developed under the above stress-strain relations is by 

iHoskins, L. M., Flow and fracture of rocks as related to structure: 16th 
Ann. Rept. U. S. Geol. Survey, pt. 1, 1896, p. 845 et seq. 



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COMPRESSION FRACTURES 



17 




Fig. 3. Results of crushing wooden blocks by non-rotational strain. Note ten- 
dency of fractures to follow shearing planes 45° to the pressure (which was from 
above) regardless of the grain of the wood. 




Fig. 4. Fracture of building stone (brown sandstone) along shearing planes. 

Buckley. 



After 



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18 



STRUCTURAL GEOLOGY 




Fig. 5. Wire netting model undeformed. See also Figs. 6 and 7. 

the use of the sphere as the unit of original structure and the strain 
ellipsoid as its deformed equivalent. Fractures under compression 
tend to follow the cross sections in the strain ellipsoid which are the 
same in dimensions as those of the original sphere; in other words, 
planes (called planes of no distortion) determined by the intersec- 
tions of the original sphere with the strain ellipsoid. 



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COMPRESSION FRACTURES 



19 



A simple device for illustrating the position of strain ellipsoid 
and shearing planes in both rotational and non-rotational strain 
is shown in Figs. 5, 6, and 7. A cardboard upon which is inscribed 
' a circle is laid between two sheets of wire netting. The three are 
then fastened together by a rivet in the center of the circle. A 
wooden hinged frame fastened to the netting allows and controls 
the distortion of the netting, while the interior sheet remains undis- 




Fig. 6. Wire netting model deformed by non-rotational strain. Straight lines 
connecting intersections of circle and ellipse mark positions of "planes of no 
distortion" or planes of maximum shear. 

torted. A circle and diameters are painted on the netting corre- 
sponding with those on the central sheet. When the screen is 
distorted the circle on the wire becomes an ellipse or a cross section 
through the greatest and least principal axes of a "strain ellipsoid," 
which is superposed upon the undeformed circle of the cardboard. 
In Fig. 6 a non-rotational strain is represented, called "pure 
shortening and elongation." The circle elongates normal to the 
pressure. The planes of no distortion, which are the planes of 



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20 



STRUCTURAL GEOLOGY 



maximum shear, stand normal to the surface of the screen. Their 
intersections with the plane of the screen are to be seen at about 
45° to the pressure. It will be noted that the lines representing the 
planes of shear are parallel to the wires. The distortion of the 
screen actually occurs by shearing of the wire mesh. This should 




Fio. 7. Wire netting model deformed by rotational strain, or shear. Straight lines 
connecting intersections of circle and ellipse mark positions of "planes of no 
distortion" or planes of maximum shear. 

make clear the fact that the painted lines of "no distortion" are 
actually shearing planes. 

In Fig. 7 the strain is a rotational one. A strain ellipse is pro- 
duced by shearing of the top over the bottom of the model, ob- 
viously by movement along the shearing planes of the wire mesh. 



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COMPRESSION FRACTURES 21 

The planes of no distortion are indicated as before. It will be noted 
that they have the same relations to the ellipse as before, though 
the pressure has been applied at a different angle. It is evident 
that the net result is the same as in a non-rotational strain, so far 
as the shape of the strain ellipse is concerned. The rotation of the 
one figure in space would make it coincide with the other. 

It cannot be too strongly emphasized that in nature what we 
usually see is the net result, which we may interpret in terms of 
strain ellipsoid. This strain ellipsoid may have been developed 
either by rotational or non-rotational strain, and we must be care- 
ful not to assign the strain ellipsoid to either kind of strain on 
insufficient evidence. There are cases where it is possible to make 
such definite assignment. 

Rock fracture tends to occur under any one of the stress-strain 
relations, or some combination of them, described above under the 
headings Tension Fractures and Compression Fractures. Initial 
planes of weakness may modify these relations. In homogeneous 
masses these are the limiting cases which cover all rock fractures. 
In the field study of fracture it is sometimes possible to determine 
what the stress-strain relations have been; commonly it is not. It 
seems to the writer, therefore, that great care should be taken in 
choosing a general nomenclature for fractures which would not 
imply a knowledge of stress-strain relations we do not possess. 

JOINTS 

It is sometimes convenient to classify joints as strike joints or 
dip joints, to indicate concisely their parallelism in direction with 
the strike or dip of beds. Joints are ordinarily classified as tension 
and compression joints to express their relations to stresses. In 
nine cases out of ten the student sees nothing in the joint itself 
which tells him whether the joint results from tension or compres- 
sion, and the attempt to use this classification may lead to un- 
warranted conjecture, or may throw him into the discouraged 
state of mind of a person who believes that he should be able to 
tell something which the facts do not readily indicate. It is 
pertinent to inquire as to what conditions tell definitely whether 
any particular system of joints is due to tensional or compressive 
stress. 



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22 STRUCTURAL GEOLOGY 

JOINTS WHICH CAN BE CLASSIFIED AS DUE TO TENSION 

(a) Faulting may imply extension of surface (see pp. 55-56), 
and hence the association of joints with such faulting would 
suggest their development by tensional stresses. 

(b) Open joints indicate tension, but it is difficult to determine 
whether tension existed at the time the joints were formed or was 
subsequent to their genesis. 

(c) Tension joints have been found along the crests of anti- 
clines, developed as indicated in the diagram (Figs. 8 and 53). 
These, however, are usually on a small scale. The writer knows of 




Fig. 8. Tension joints on anticline. After Van Hise. 

no cases described for the United States in which any regional set 
of joints has been positively related to tensional stresses developed 
along major anticlines, but the existence of such cases is reasonably 
inferred where joints are parallel to the axial planes of folds. 

(d) During the process of cooling in igneous rocks, tensional 
stresses are set up in them; and these stresses result in the forma- 
tion of joints, not only in the igneous masses themselves, but in the 
adjacent rocks. The remarkably complicated fractures of Tonopah 
and other mineral-bearing districts of the Great Basin first sug- 
gested this origin, and it seems to be now an established fact that 
much of the complex fracturing of igneous rocks may be related 
definitely to their cooling. (See page 43.) Such joints may not 
be persistent or in regular systems. Locally the fractures take 
certain curved or concentric forms about loci of cooling, as for 
instance, in the gabbro of the Cobalt district of Ontario, or in the 
slates with which the gabbro has come into contact. These slates 
have been heated and caused to expand under the influence of the 
intrusive and have subsequently cracked on loss of heat. Basaltic 
parting is only a special type of tension jointing developed by 



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TENSION JOINTS 23 

cooling. Radial and peripheral fractures seem in some cases to 
have been developed by the cooling of laccoliths and batholiths. 
Laccoliths have sometimes been supposed to pull away from the 
walls in the manner of a cooling melt from a mold, as for instance, 
in the Iron Springs district of Utah. 

(e) Another type of local tension jointing is developed by. the 
drying out of a sediment, resulting in the formation of mud cracks; 
or the desiccation of sediments on a large scale. The joints so 
formed lack regularity and persistence. It is possible that many of 
the fairly extensive joints in flat-lying sedimentary beds like the 
Paleozoic of the Mississippi valley may be due to the drying and 
settling of the formation. 

(f) In some cases where dominant joints can be identified as the 
result of shearing stresses, as for instance, in a shaly layer sheared 
between two hard quartzite beds, small tension gash joints have 
been an incidental development. (See pp. 16 and 25). 

JOINTS WHICH CAN BE CLASSIFIED AS DUE TO COMPRESSION 

(a) Compressive joints may be sometimes identified by evi- 
dences of slipping, such as slickensides, developed along the joint- 
ing planes; but these evidences do not necessarily indicate that the 
compressive stresses were applied at the time the joints were 
formed. 

(b) Where these joints pass into overthrust faults or folds, as, 
for instance, in the southern Appalachians, they are likely to be 
compression joints. 

(c) Compressive joints may also be identified frequently on the 
limbs of folds by the manner in which they follow closely the 
theoretical directions required for compressive shear by the stress 
conditions occurring at those places (see pp. 20-21). For instance 
in the Baraboo quartzite in Wisconsin (see Figs. 9 and 11), there 
are joints parallel to the bedding, along which there has been a 
slight amount of slipping; there is another set inclined to the bed- 
ding; this latter set is continuous in direction only through homo- 
geneous beds and passes to other beds by an offset or a curve along 
the bedding planes. In the softer beds the joints are so closely 
spaced as to yield a "fracture cleavage" (see p. 63). The joints 
have positions accordant with the supposition that they have been 



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Fig. 9. Fracture cleavage and jointing developed by shearing between beds in Baraboo 
quartzite. After Atwood. The light portion on the right is a bed of brittle quartz- 
ite. The dark portion on the left is a bed of softer shaly quartzite. The outcrop is 
a part of the north limb of a syncline. The right hand bed is on the south. It has 
obviously moved upward with reference to the beds to the north of it, as would be 
expected from this position on the syncline. The fractures here have been de- 
veloped by rotational or shearing stresses described on pp. 16, 20. It is suggested 
that the student superpose on these beds the theoretical positions of the strain el- 
lipsoids and the planes of maximum shear. Note relations of fracture cleavage to 
jointing in adjacent bed. (See also page 121). 

24 



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COMPRESSION JOINTS 



25 



formed by compressions 1 shearing, caused by slipping between the 
beds. Short open gashes or joints are also developed here by ten- 
sion, as indicated in the figure. 

(d) The sheet structure so commonly observed and utilized in 
granite and other quarries is a system of jointing probably at least 
in part developed under compressive stresses. (Figs. 12, 13 and 14.) 







£**£ 


- I 






<Jt 




i 








1 


I; ~. J^B' ^l^SHH 






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Fig. 10. Fracture cleavage developed in slaty quartzite layer between two massive 
beds of quartzite, on south limb of the Baraboo syncline, Wisconsin. Note the 
direction of differential movement and correlate this with position on the fold. 
W r hat are the relations of the cleavage to pressure? Note relations of fract- 
ure cleavage to joints in the adjacent massive layers. (See also Fig. 37 and 
page 121). 

The sheets are thinnest near the surface and rapidly thicken below. 
They may be curved, and in general are parallel with the rock sur- 
face. Usually they are found to be lens-shaped when traced some 
distance. Many instances have been noted of a lengthening of 
blocks when quarried out, sometimes with explosive violence, 
indicating that in the ledge they were under compressive stress. 



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26 



STRUCTURAL GEOLOGY 



Compression is indicated also by the occasional flattening, by 
faulting, of drill holes and other openings. 1 The sheet structure is 
developed artificially by the use of explosives, by hot air, and by 
heating the surface. These compressive stresses have been re- 
ferred to various causes — solar heat, weathering (or kaoliniza- 
tion), expansion of the surface due to removal of overlying load 
by erosion, and to major earth movements. 2 

> South 




Fig. 11. Vertical section Baraboo quartzite, normal to the strike, on the South 
Range, Baraboo district, Wisconsin, showing joints formed by the folding of 
weak, thin beds interstratified with thick, strong beds. After Steidtmann. 
Short open gashes or tension joints may be seen crossing the curved compres- 
sion joints in the softer layers. 

Whatever the cause, the upper layers tend to extend themselves 
farther than the lower layers by shearing, producing the sheeting 
planes between them. The same structure has been referred also 
to tension due to cooling of the igneous rocks while still under 
sedimentary load, the sheets being approximately parallel to the 

1 Dale, T. Nelson, The granites of Vermont: Bull. 404 U. S. G. S., 1909, pp. 17-18. 

2 Idem; also Bulls. 354 and 484, U. S. Geol. Survey. 



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COMPRESSION JOINTS 



27 



original contact surface of the intrusive. Bearing in mind the 
parallelism of the sheets to the present erosion surface, and their 
diminution in number below the surface, the explanation of tension 
by cooling involves the assumption that the present erosion sur- 




Fig. 12. Sheet structure in granite. After Dale. 

face is nearly the same as the original contact surface, which cer- 
tainly is not always true. 

The sheets are crossed by vertical joints which partly result from 
tension due to gravity acting on the thin sheets. Some of them also 
may be compressive. By application of the principles of breaking 
under rotational or shearing strain given above, it will appear 
that a complementary set of compression fractures should be 



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28 



STRUCTURAL GEOLOGY 



expected approximately at right angles to the sheeting planes. 
In quarries these vertical joints may be in one or more intersecting 
«ets. They are characteristically intermittent, extending through 
a given set of sheets and offsetting in the sheets above and below. 
Not infrequently they are curved. 






Fig. 13. Spalling of surface by shearing due to heating or cooling. After Van Hise 
(a) shows the condition of a block of uniform temperature, (b) illustrates the 
manner in which the upper portion of a rock surface expands when heated 
above average temperature; where the difference in temperature is sufficiently 
great, this results in the splitting off of the upper layers, (c) illustrates the 
contraction of the upper surface by cooling below the average temperature; 
where the difference in temperature is sufficiently great, this results in the 
splitting off of the upper layers. 

JOINTS DEVELOPED UNDER UNKNOWN STRESS-STRAIN 
CONDITIONS 

Probably the great majority of joints has not yet been satisfac- 
torily determined as belonging to the tension or compression class. 
-Many instances might be cited of attempted classification without 



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JOINTS WIDENED BY GROWING CRYSTALS 29 

sufficient proof. Especially numerous have been the attempts 
to classify joints as compressive when they are in vertical inter- 
secting sets, on the assumption that the intersection of the joints is 
an indication of compression stresses. On the other hand, identi- 
cally similar sets of joints have been referred to tension acting in 
mutual perpendicular directions, or to torsion in the manner 
indicated by Daubree's experiment. (See p. 15.) 

WIDENING OF JOINTS BY THE LINEAR FORCE OF GROWING 

CRYSTALS 

It has long been known that crystals exert very considerable 
force in growing. Crystals of pyrite, for instance, drive apart the 
laminae of slates. Experiments on the pressure exerted by growing 
crystals of alum and other salts have shown that they exert a 
pressure of the same order of magnitude as the ascertained resist- 
ance which the crystals offer to crushing stresses. 1 It is supposed 
that this force exerted by crystals may be a factor in widening 
mineral-filled fissures, like the gold-bearing quartz veins of the 
Mother Lode of California, some of which have a width of several 
hundred feet. This width is not observed in unfilled fissures. 
In fact, the unfilled fissures are in general very narrow as com- 
pared with the fissures which have been filled and cemented. 
According to Becker, 2 laminae of the slates on two sides of Mother 
Lode veins have locally been driven apart and contorted. He con- 
cludes that when such occurrences cannot be accounted for by 
faulting the inference is almost unavoidable that the laminae have 
been driven apart by the force of growing crystals of quartz, the 
axes of which stand sensibly at right angles to the planes of the 
laminae. The ribbon ore, consisting of parallel laminae of slate, 
separated by quartz, has been regarded as due to faulting, but 
evidence of faulting is often lacking and it is difficult to conceive 
how faulting could separate these slate bands so evenly. Separa- 
tion by the growing force of quartz crystals is an alternative ex- 
planation. 

If quartz during crystallization exerts a pressure on the sides 
of the vein which is of the same order of magnitude as the resist- 

1 Becker, G. F., and Day, Arthur L., The linear force of growing crystals: Proc. 
Wash. Acad. Sci., Vol. 7, 1905, pp. 283-288. 

2 Op. cit., p. 284. 



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30 



STRUCTURAL GEOLOGY 



ance which it offers to crushing, as Becker 1 thinks probable, then 
this force is also of the same order of magnitude as the resistance 
of wall rocks, and thus it becomes possible that the widening of the 
filled fissures may be largely due to this cause. 

SURFACE EXPRESSION OF JOINTS 

What has already been written about the surface expression 
of the zone of fracture applies specifically to joints. One need only 
cite the Grand Canyon, Yosemite Valley, or the Dells of the Wis- 




Fiq. 14. Spalling of andesite outcrops, presumably due to alternate heating and 
cooling in weathering. 



consin river, where the drainage has been controlled almost en- 
tirely by joints. In other areas, especially in drift-covered areas, 
the relation may be a very slight one. In some cases the assump- 
tion of relationship has been carried so far that drainage lines have 
been taken as evidence of joints without further information, and 
continuity and regularity of joint systems have been assumed on a 
basis of too little information. Attention has been called above to 
joints of wide distribution which characteristically lack regularity. 

1 Op. cit., p. 287. 



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LABORATORY STUDY OF JOINTS 31 

SUGGESTIONS FOR LABORATORY WORK ON JOINTS 

(See also pp. 60-61) 

On the experimental side the suggestions made in connection with 
faults on pages 60-61 apply equally well to joints. Much can be done 
with maps of joints. In this connection it is to be remembered that it is 
the interpretation of joints that is wanted and not. a mere description of 
joints. 

It is suggested that the student study the joints of the areas named 
below and make the attempt to classify them as tensional or compres- 
sional; and if compressional to discriminate between rotational and non- 
rotational strains. He should not go farther in inferences than the facts 
warrant. If he becomes satisfied of the origin of certain joints he should 
not assume that all joints in this area have the same origin. Inferences 
from the facts should be drawn regardless of what is said about the joints 
in the accompanying reports. 

11 Joint system in the rocks of southwestern Wisconsin and its relation to the 
drainage network 11 by Edmund Cecil Harder, Bulletin of University of Wis- 
consin, Science Series, Vol. 3, No. 5. The joints here described are fairly 
typical of the joints of the flat-lying Paleozoic beds of the Mississippi 
valley. Careful reasoning from the facts will eliminate certain hypotheses 
of the origin of these joints and point with a reasonable certainty to the 
true origin. Somewhat similar conditions in the Grand Canyon and 
Yosemite Valley should be studied; also the Watrous, New Mexico, topo- 
graphic map. The relation between topography and jointing, due to 
interaction of climate, rock structure, and lithology, is to be noted. 

11 The secondary structures of the eastern part of the Baraboo quartzite 
range, Wisconsin" by Edward Steidtmann, Journal of Geology, Vol. XVIII, 
No. 3, 1910. The problems of jointing in folded rocks are here illustrated 
and discriminated with unusual clearness. 

" Granites of Maine, Massachusetts, New Hampshire, Rhode Island, and 
Vermont" by T. Nelson Dale, Bulletins 313, 354, 404, U. S. G. S. The 
jointing of igneous rocks is here admirably illustrated and discussed. 
Before reading Dale's discussion of origin, the facts of jointing which he 
describes should be carefully considered and an attempt made to formulate 
a reasonable hypothesis of origin to fit these facts. Then compare with 
Dale's conclusion. 

FAULTS 

Faults are fractures along which there has been some relative 
displacement of the rocks. They differ from joints mainly in the 
extent of the displacement and in the emphasis on the displacement 
parallel to the plane of fracture rather than normal to it. All 
fractures are accompanied by some displacement — in fact, frac- 



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32 STRUCTURAL GEOLOGY 

tures would not occur were not some displacement required by the 
stresses. 

NOMENCLATURE 

The elements of a fault are: hade, or the angle made by the 
fault plane with the vertical; dip, or the angle made by the fault 
plane with the horizontal; throw, or the displacement of the beds 
measured parallel to the dip of the fault plane; and the heave or 
shift, or the displacement of the masses measured parallel to the 
strike of the fault plane. When a fault plane dips toward the 
downthrow side, the fault is called a normal or gravity fault 
(Fig. 17). The displacement of the crust by such faults is ap- 
parently downward and therefore apparently due to gravitational 
forces. Where the fault plane dips toward the upthrow side of the 
fault, the fault is called a reverse or thrust fault (Fig. 18). The dis- 
placement of the crust is then apparently of the nature of tangen- 
tial shortening. Normal faults may result in the dropping of 
blocks called graben. These usually have polygonal outlines. 
Blocks standing up between graben are called horsts or bridges. 
A fault with vertical displacement is expressed at the surface as a 
small cliff or scarp to which the name " fault scarp" has been given. 
" Fault trace," " furrow," and "rift" are terms given to the line of 
intersection of the fault plane with the surface. They are es- 
pecially used where the fault displacement is horizontal and there 
is no fault scarp, or where the fault scarp has been worn down by 
erosion. 

It will be noted that the classification of faults into normal or 
gravity and reverse or thrust faults, takes account only of apparent 
relative displacement in a vertical section normal to the fault plane. 
It takes no account of horizontal or oblique displacement. It 
expresses merely the present relations of the beds in a two dimen- 
sional cross section rather than in three dimensions. It tells us 
nothing of the actual displacements of the beds. Hinge or pivotal 
faulting about an axis normal to the plane of faulting may produce 
a fault which on one side of the pivotal axis would be called normal 
and on the other side reverse, and yet there may not be any differ- 
ential movements in the centers of the mass of the two parts of 
the faulted body (Fig. 23). A purely horizontal displacement may- 
appear either as a normal or reverse fault at any one place, de- 



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NOMENCLATURE OF FAULTS 



33 




Fig. 15. Perspective view and vertical section of a thrust fault. After Willis. 



V. 




Fig. 16. Diagram of a thrust fault. After Willis. 



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34 



STRUCTURAL GEOLOGY 



pending upon the attitude of the beds with regard to the plane of 
the fault (Figs. 19, 20, 21, 22, 24, 25). 




Fig. 17. To illustrate relative positions of blocks in normal or gravity faulting. 

In general we have attempted to use too simple a nomenclature 
by which to classify faults. The classification is inadequate to 
give any accurate description of the great variety of relative dis- 




Fig. 18. To illustrate relative positions of blocks in thrust or reverse faulting. 

placements possible along a fault plane. The inadequacy of 
the old method has been realized in recent years by many workers 
in the field, especially by men who have found it necessary to work 



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NOMENCLATURE OF FAULTS 35 




Fig. 19. Normal faulting produced by horizontal movement along table top. 




Fig. 20. Reverse or thrust faulting produced by horizontal movement along 

table top. 



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36 STRUCTURAL GEOLOGY 

out in detail the extremely complicated fault systems in rocks 
associated with certain ore deposits. Consequently there have 
been several attempts to develop a more adequate nomenclature 
to describe the great variety of conditions met with in the field. 
Some of these attempts are very elaborate. There is yet no general 
agreement on any of these schedules. 1 Therefore none of these 
classifications will be given here. It may be questioned whether 
special nomenclature of faults is necessary. There has been 
developed in mechanics a technical nomenclature to describe dis- 
placements of all solid substances, which may as well be used for 
faults as a long series of names, difficult to remember, coined for 
the special use of the geologist. Quoting from Chamberlin 2 : 
The complaint "that our predecessors have trammelled us with 
premature and ill-chosen classes and names has for its logical 
response a forbearance on our part from further imposition of the 
kind on our successors; perhaps also it suggests an effort to free our- 
selves from our hamperings by dropping embarrassing terms, and 
by de-technicalizing such as it seems best to retain." 

The terms "normal fault" and "thrust or reverse fault" have 
become so well intrenched in the literature of the subject that it is 
difficult to avoid their use. The writer believes that the two terms 
should be retained to express apparent displacements in a plane of 
section normal to the fault plane — not as implying real displace- 
ment. This restricted usage involves no wide departure from that 
of the past, but it emphasizes that which has too often been over- 
looked, i. e., that the terms have reference essentially to displace- 
ments as they appear in a two dimensional cross section. 

APPARENT AND REAL FAULT DISPLACEMENTS 

Fault displacements shown in a two dimensional cross section 
should be assumed to be apparent until the actual displacement has 
been proved. Arrows, commonly used to indicate displacements 
upon cross sections, are often misleading. They show only the 

1 Jaggar, T. A., Jr., How should faults be named and classified? Econ. Geol., 
Vol. 2, 1907, pp. 58-62; Spurr, J. E., idem, pp. 182-184, 601-602; Willis, Bailey, 
idem, pp. 295-298; Cushing, H. P., idem. pp. 433-435; Tolman, C. F., Jr., idem, 
pp. 506-511; Evans, John W., idem, pp. 803-806; Chamberlin, T. C, The Fault 
Problem, idem, pp. 585-601, 704-724. 

2 Econ. Geol., Vol. 2, 1907, p. 585. 



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FAULTS 



37 




Fig. 21. Thrust fault relations produced by horizontal movement. 




Fig. 22. Normal fault relations produced by horizontal movement. 



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38 STRUCTURAL GEOLOGY 

apparent displacement in the plane of the cross section. They are 
likely to be assumed to show the real displacement. 

Until the real displacement is actually proved, we cannot avoid 
the consideration of any of the possibilities. The limiting cases 
have been discussed on preceding pages. Yet seldom indeed do we 
keep a sufficiently open mind in this regard. To illustrate, in 
the southern Appalachians where there are repeated overthrust 
faults associated with overthrust folds, the structural facts are 
commonly shown on vertical sections normal to the trend of the 
fault traces or of the mountain ranges. Without analyzing the 
possibilities, we are likely to assume that the shortening is in 
the plane of the cross section, and may overlook the fact that the 
apparent displacement shown in the cross section may not be the 
real displacement and that the same structural features might have 
been produced by a couple of forces acting in directions inclined to 
apparent shortening, producing a shearing movement. If a de- 
formed area of this type be regarded as a whole as a strain el- 
lipsoid (see pp. 16-20) with its longer dimensions parallel to the 
trend of the range, there is perhaps less difficulty in realizing that 
the deformation may have been accomplished either by pure short- 
ening or by shearing (see pp. 19-20), or more probably by some 
combination of the two limiting cases. 

Even the use of the strain ellipsoid may be misleading, if care 
is not taken to ascertain whether the longest principal axis is 
vertical or horizontal or inclined. For instance, any ellipse super- 
posed upon the Appalachian area with its longer axis parallel to 
the range is a cross section of an ellipsoid. It must not be as- 
sumed that the longer axis of this ellipse is really the greatest 
principal axis of the ellipsoid. In other words, the extension may 
have been greater upward than along the trend of the range. If 
it is true that fractures develop along the planes of no distortion in 
a strain ellipsoid and that the thrust faults of the Appalachians are 
controlled by this law, then it follows that the longest axis of the 
strain ellipsoid must have been essentially vertical. This is a 
natural expectation, for there are reasons to believe that the 
relief has been easier upward than laterally. 

With alternative hypotheses open, how may the actual dis- 
placement be ascertained? It is frequently impossible to do 
this but there are certain ways in which the actual displacement 



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DETERMINATION OF REAL DISPLACEMENTS 39 

has in some localities been worked out. These ways are as 
follows: 

Striations may mark the direction of displacement. In some 
cases in repeated movements later striations have destroyed 
earlier ones, perhaps formed in different directions. 

The matching of the ends of broken dikes often makes it possible 
to determine the actual displacement of faults. This method has 
been used very effectively by the geologists of the Geological 
Survey of Scotland in determining both the direction and amount 
of the displacement of some of the large overthrust faults of the 
northwest Highlands of Scotland. Careful petrographic discrimi- 
nation of these dikes and their uniformity and trend has aided 
greatly in tracing the dikes individually and in sets. 

The matching of displaced ore-bearing veins has often indicated 
the actual displacement of faults. Probably in few other cases 
have the displacements of faults in three dimensions been con- 
sidered so carefully as they have in many mining camps. The 
student is referred to Weed's monograph on the Butte district 1 
and Emmons 1 and Garrey's bulletin on the Bullfrog district 2 for 
quantitative studies of actual fault displacements. 

NORMAL FAULTS 

Under this heading are considered faults in which the apparent 
displacement is downward on the overhanging side. In some cases 
the apparent displacement is known to be the real displacement — 
in other cases it is not. 

Ordinarily a normal or gravity fault is regarded as the expres- 
sion of tension, and a reverse or thrust fault as evidence of com- 
pression; but, as noted under the preceding headings, the elonga- 
tion and shortening expressed by the terms normal faulting and 
reverse faulting have reference to the relations of the beds ex- 
pressed in a plane normal to the fault plane. When considered in 
three dimensions, normal faulting may not show any extension of 
the mass as a whole, and reverse faulting may not show any 
shortening of the mass as a whole. Hinge faulting about an axis 
may produce on one side normal fault relations and on the other 

1 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. 
Paper U. S. Geol. Survey No. 74, 1912. 

2 Ransome, F. L., Emmons, W. H., and Garrey, G. H., Geology and ore deposits 
of the Bullfrog district, Nevada: Bull. 407, U. S. Geol. Survey, 1910. 



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40 



STRUCTURAL GEOLOGY 



reverse fault relations although there may have been no differential 
movements of the centers of mass of the two parts of the faulted 
body. Horizontal movement alone may result in apparent normal 
and reverse faults. (See Figs. 19, 20, 21, 22, 24, 25.) 

Also faults which may prove to be tension phenomena may 
be merely subsidiary expressions of a major compressive thrust. 
Sometimes it is necessary to know only the actual displacement of 




Fig. 23. To illustrate hinge faulting. This would appear as a normal or gravity 
fault on a plane normal to the fault plane passing through the ends of the blocks 
nearest the reader and as a thrust or reverse fault in a plane passing through the 
ends of the blocks farthest from the reader. 

a minor portion of the faulted mass, as for instance, in a mine, 
regardless of any relation to major deformation. Ordinarily, 
however, it is desirable to relate the minor faulting to major 
deformation, and this is a much more difficult problem. An ex- 
treme case cited by Chamberlin 1 is that of a fault passing through 
the slope of a hill and displacing talus blocks. Knowledge of the 
relative displacement of the blocks in the talus slope may give 

1 Op. cit., p. 589. 



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NORMAL FAULTS 



41 



little clue to the major and controlling displacement. In almost 
any complexly faulted area the local displacements may be varied 
and yet the major and controlling displacement be a comparatively 
simple phenomenon. A great thrust fault resulting in uplift may 
be accompanied by a considerable variety of local displacements 
which would be interpreted locally as both thrust and tension 
faults. These are subsidiary and local phenomena due to relaxa- 




Fiq. 24. Block dislocated by heave fault, showing apparent reverse faulting of 
bed BB. After Ransome. 

tional movement, to the concurrent action of gravity, and to 
other causes. 

Whether a given fault is really tensional or compressional when 
considered in three dimensions, whether it is subsidiary to a major 
fault of different displacement, has been satisfactorily determined 
in comparatively few instances. While the terms tension and 
compression are freely applied to faults, this is really done on the 
unreliable assumption that the apparent displacement in a vertical 
plane represents the actual displacement. 

Means of identifying tension joints discussed on pp. 22-23 may 
be used also for determining local tension faults. 



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42 



STRUCTURAL GEOLOGY 



Normal Faults Associated vnth Igneous Rocks: — Faults are likely 
to be numerous within and adjacent to areas of igneous activity. 
They are especially numerous in surface volcanics. Such faults are 
more or less irregular and discontinuous, and offset along cross 
faults and joints, breaking the rocks into heterogeneous polygonal 
blocks. Displacements are both horizontal and vertical. Normal 

Z B 




Fiq. 25. Bloek dislocated by movement between heave and upthruat, showing 
apparent normal faulting. After Ransome. 

faults predominate. Hinge faults are not uncommon. These 
faults are well illustrated on many maps of western mining districts 
prepared by the U. S. Geological Survey, notably those of the Ton- 
opah, 1 Goldfield, 2 Bullfrog, 3 and Clifton 4 districts. 

1 Spurr, J. E., Geology of the Tonopah Mining District, Nevada: Prof. Paper 
No. 42, U. S. Geol. Survey, 1905. 

2 Ransome, F. L., Geology and ore deposits of Goldfield, Nevada: Prof. Paper 
No. 66, U. S. Geol. Survey, 1909. 

8 Ransome, F. L., Emmons, W. H., and Garrey, G. H., Geology and ore deposits 
of the Bullfrog district, Nevada: Bull. 407, U. S. Geol. Survey, 1910. 

4 Lindgren, Waldemar, Copper deposits of the Clifton-Morenci district, Arizona : 
Prof. Paper No. 43, U. S. Geol. Survey, 1905. 



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NORMAL FAULTS 43 

It has long been suspected that there is some genetic connection 
between faulting and igneous activity. Spurr expressed this 
specifically as follows: * "It is plain that the faulting was the 
result of adjustments of the crust to suit violent migrations of 
volcanic rock; that it originated with the swelling up of the crust 
and its forcible thrusting up and aside to make way for the numer- 
ous columns of escaping lava; and that after the cessation of the 
eruptions it was continued by the irregular sinking of the crust 
into the unsolid depths from which the lavas had been ejected. It 
can readily be seen that all sorts of pressure (from below upward, 
lateral, and downward, by virtue of gravity) must have been 
concerned in such movements, and that the first faults were due 
rather to upward and lateral irregular thrusts, while the later 
ones (in many cases along the same planes as the first) were due 
to gravity. So reversed and normal faults are equally natural, and 
both occur frequently." 

"The writer at first looked upon the faulting at Tonopah as 
exceptional and local, and not to be connected with ordinary 
faulting in the Great Basin, but there now appears no reason for 
doubting that the phenomena within this small, carefully studied 
area are typical of the unstudied similar volcanic region beyond 
the limits of the map." 

In discussing joints, attention has been called to the common 
development of joints and partings during the cooling of igneous 
rocks, including peripheral, radial, concentric, basaltic, and ir- 
regular partings. Faulting may follow any of these surfaces of 
weakness. 

Normal Faults in Unfolded Sediments: — Normal faults may be 
locally developed in nearly flat-lying sediments. Here the cause of 
tension may be shrinkage and settling due to drying and recrystall- 
ization. Often no other causes are discernible, but it is not possible 
to exclude hypotheses of regional or deep-seated tension related to 
major earth movements. 

Association of Normal Faults with Folds: — The reconstruction of 
an area with abundant normal faults may develop a fold or dome of 
low slope, suggesting that the normal faults result from the action 
of gravity upon a mass elevated by folding but inadequately sup- 
ported. Normal faults associated with overthrust folds, to be 

1 Op. cit., p. 80. 



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44 STRUCTURAL GEOLOGY 

seen, for instance, in the southern Appalachians, seem to be the 
natural consequence of settling following disturbance of equilib- 
rium by thrust, in other words, of relaxation so commonly fol- 
lowing compression. 

The attempt has been made also to correlate tension faults 
existing over a great area with the collapse of a very gentle arch. 
For instance, the great normal faults in the Great Basin area are 
referred to the collapse of an arch originally extending from the 
Wasatch on the east to the Sierra Nevadas on the west. 1 Where 
broad, gentle arches are thrown up through compression or through 
changes in support below, the inherent weakness of the rocks may 
cause them to break almost from the start and allow certain blocks 
to settle within the arch. Chamberlin 2 has called attention to the 
inherent weakness of rocks and their inability to support them- 
selves in large masses. For instance, a dome 80 miles in diameter, 
of any thickness, with the curvature of the earth, will bear only 1 / 48 
of its own weight. It is therefore apparent that when any great 
earth movement is initiated, tending to arch any part of the earth's 
surface, unless this arch is thoroughly and evenly supported by 
great masses below, it will be unable to sustain itself by its own 
strength alone; and one would expect a settling of blocks, giving 
the tension or normal type of faulting and jointing, with conse- 
quent extension of surface. 

No such relation as that discussed in the above paragraph has 
been proved on any large scale. The existence of such a primary 
arch or tendency for arching is inferred as a possibility from the 
existence of supposed tension faulting. 

Vertical and Steeply-Dipping Normal Faults and Joints in Inter- 
secting Systems: — While thrust faults with low dips are frequently 
related to overthrust folds and have been usually ascribed to 
horizontal compression, in many cases such pressures have also 
been held responsible by some geologists for intersecting systems 
of vertical or steeply-dipping faults and joints. 3 Becker 4 called 
attention to the fact that it is mechanically possible for inter- 

1 Gilbert, G. K., Report on the geology of portions of Nevada, Utah, California, 
and Arizona, examined in the years 1871 and 1872: U. S. Geog. Surveys W. 100 th 
Mer., Vol. 3, 1875, pp. 54-56. 

2 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. 1, 1904, p. 555. 

3 Hobbs, W. H., The Newark system of Pomperaug Valley, Conn.: 21st Ann. 
Rept., U. S. G. S., pt. 3, 1901, pp. 7-162. 

4 Becker, Geo. F.: Bull. Geol. Soc. Amer., Vol. 4, 1893, p. 50. 



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NORMAL FAULTS IN INTERSECTING SYSTEMS 45 

secting vertical or steeply-dipping faults and joints to develop 
under horizontal compressive stresses only when lateral relief were 
easier than upward relief, and that such conditions prevail over 
certain areas. (See pp. 28-29) Many other geologists ap- 
parently have not analyzed the subject and have overlooked this 
qualification of Becker's, having assumed that the fact of inter- 
section in sets implies compressive stresses, without considering 
alternative hypotheses. 

The possibilities of lateral relief rather than upward relief are 
difficult to determine in the field, and ordinarily it is not possible to 
be sufficiently certain about these to. draw inferences from them as 
to the origin of the fault by tension or compression. Irregularities 
of surface, like valleys, may permit easy lateral expansion in inter- 
vening ridges, thus allowing the formation of vertical intersecting 
faults or joints by compressive stresses. However, in these cases it 
is altogether likely that, for each vertical fracture, the comple- 
mentary fracture (see pp. 27-28) may be a horizontal shearing 
fracture rather than another vertical one, and that the existence of 
intersecting vertical fractures may be due to tension acting simul- 
taneously or successively from two or more horizontal directions. 
The intersecting vertical joints then have purely fortuitous rela- 
tions. They do not intersect at definite angles determined by the 
shearing stresses. 

On the assumption that vertical or steeply dipping joints and 
faults are the result of tension, there have been two explanations to 
account for their existence in intersecting sets or systems. One 
explanation is that they were developed by torsion (see page 15). 
The other explanation is that they are the results of successive 
earthquake shocks from different directions, in which case they 
form under the tensional component of the wave, normal to the 
direction of propagation of the earthquake wave (see pp. 67-68). 
Still other explanations are possible. Joints and faults formed by 
the cooling of an igneous mass, or the settling and drying of a 
sediment may be in more or less regular sets. Relaxational settling 
after a period of compressive faulting or folding may develop nor- 
mal or steeply dipping joints and faults in intersecting sets. The 
systems in these cases are not likely to be uniform and yet for small 
areas may have a considerable regularity of arrangement. 

Another explanation is that one of the vertical sets may be 



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46 



STRUCTURAL GEOLOGY 



tensional and the intersecting set may be compressional, in the 
manner determined experimentally (see p. 16). 

REVERSE OR THRUST FAULTS 

Sections in the southern Appalachian folios show thrust faults 
associated with overthrust folds. The fault planes may be in- 
ferred to have the relations to stress indicated on pp. 20-21, in 
rotational or shearing compressive strains. The inference is 
usually made that there is an overthrust, and therefore shortening, 
of so many feet. This is true in the plane of the section. It tells 
us nothing of the movements inclined to the plane of the section, 




Fig. 26. Overthrust faulting localized by tension fracture "break thrust." After 
Willis. 1. Shows break in the massive limestone bed which determines the 
plane of the break thrust along which the displacement shown in 2 takes place. 

which may have been fully as great. The association of "thrust" 
faults with overthrust folds usually indicates compression, but not 
so much compression as a two-dimensional cross section might 
indicate. Consideration of many cross sections is the same in effect 
as considering the fault in three dimensions, and leads to closer 
estimates of actual shortening. 

An examination of the United States Geological Survey folios 
brings out this interesting fact that in the southern Appalachians 
83% of the thrust faults, as indicated on the cross sections, are 
definitely related to overthrust folds. Willis 1 classifies them as 
(1) break thrusts where the thrust fault plane follows a previously 
formed tension fracture on the crest of the anticline; (2) shear or 
stretch thrusts, when the break follows the sheared and stretched 

1 Willis, Bailey, Mechanics of Appalachian Structure: 13th Ann. Rept. U. S. 
G. S. f pt. 2, 1893, pp. 222-223. 



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47 





Fig. 27. Illustrating thrust fault developed by stretching and by erosion. After 
Willis. 1. Stretch thrust developed from an overturned fold by stretching 
of the middle limb; 2. Erosion profile and section of a simple anticline; 3. Ero- 
sion thrust developed from the condition shown in 2 by compression from the 
plateau side, accompanied by continued erosion. 



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48 STRUCTURAL GEOLOGY 

Distributive Thrud Fault,-— \ n t . . ^ 

place along several P^c^p^Z^ ^ ^ 
rather than in a single fault plane The^elatbS Tsf " *"* 

Tl, ^ has Jn .SSfi^^ K ^ 




Fia. 28. Fault-slip cleavage in gneiss from southern Appalachians n,» • t 
been closely crcnulated and the minute folds mav r» „K<,~' Pf 8ne,ss has 
m.nute faults which now represent planes of S^ de~ ^T? ^ 
may have been cemented or may have been welded hvZt?t S The fau,ts 
to the faults there has also been d^^T^^J^ m,n - Para,1 el 
mineral particles, perhaps due in part to The slipX » L "*° gemen * ° f the 
and it is exceedingly difficult to distinguish between th *<W* ^ plaDes - 
the flow cleavage. between the fracture cleavage and 

the "Schuppen "structure of the Germans. It is well illustrated 
m the southern Appalachians. Fig. 28 shows some of these dE 
tributive faults associated with minute overthrust folds DistriK. 
ut.ve faults may be on a minute scale, a dozen of them bein ff seen 
in a single hand specimen, or on an indefinitely larger scale. Inspec- 
tion of the Roan Mountain folio of the U. S. Geological Survey of 
eastern Tennessee and western North Carolina shows a remarkable 



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DISTRIBUTIVE THRUST FAULTS 49 

series of parallel faults which on a large scale can be regarded as 
distributive faults. 




nnnnmnK 

Fig. 29. Major fault plane or fault sole. After Cadeil. 

In the northwestern Highlands of Scotland is a similar phenome- 
non, there called imbricate or schuppen structure. The fault planes 
are in shearing planes formed by compression in a rotational strain 
(see pp. 16-21). The beds are minutely sliced and piled one 
on top of the other. As the deformation continues these beds 
may ride forward as a group over a major fault plane at the bot- 
tom, sometimes called the "sole." The reports and maps of the 
British Geological Survey 1 on the Scottish highlands afford an 
unrivaled opportunity for the study of faults of this type. Experi- 
mental reproductions of these faults by Cadeil 2 throw light on the 
process. (Fig. 29). Some of his conclusions are quoted: 

1. Horizontal pressure applied at one point is not propagated 
far forward into a mass of strata. 

2. The compressed mass tends to find relief along a series of 
gently-inclined thrust-planes, which dip towards the side from 
which pressure is exerted. 

3. After a certain amount of heaping-up along a series of minor 
thrust-planes, the heaped-up mass tends to rise and ride forward 
bodily along major thrust-planes. 

4. Thrust-planes and reversed faults are not necessarily devel- 
oped from split overfolds, but often originate at once on application 
of horizontal pressure. 

5. A thrust-plane below may pass into an anticline above, and 
never reach the surface. 

1 Peach, B. N., Home, John, Gunn, W. f Clough, C. T., and Hinxman, L. W., 
The geological structure of the northwesi highlands of Scotland, with petroiogical 
chapters and notes by J. J. H. Teall, edited by Sir Archibald Geikie: Mem. Geol. 
Survey of Great Britain, 1907. 

2 Op. cit., pp. 473-176. 



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50 STRUCTURAL GEOLOGY 

6. A major thrust-plane above may, and probably always does, 
originate in a fold below. 

7. A thrust-plane may branch into smaller thrust-planes, or 
pass into an overfold along the strike. 

8. The front portion of a mass of rock being pushed along 
a thrust-plane tends to bow forward and roll under the back 
portion. 

9. The more rigid the rock, the better will the phenomena of 
thrusting be exhibited. 

10. Fan-structure may be produced by the continued compres- 
sion of a single anticline. 

11. Thrust-planes have a strong tendency to originate at the 
sides of the fan. 

FAULTS WITH HORIZONTAL DISPLACEMENTS 

The faulting in which the California earthquake originated 
followed a vertical plane along which the rocks were horizontally 
displaced. This is one of the few cases of definitely proved hori- 
zontal displacement. 1 Illustrations on a much smaller scale may 
be found in the faulting of the igneous rocks of many western 
mining districts, cited on pages 42^3. Striations on fault surfaces 
not uncommonly show that there has been some degree of horizon- 
tal displacement, even though the major displacement is vertical. 
More attention is now given than formerly to possibilities of 
horizontal displacement, with the result that more information in 
regard to this type of movement is becoming available. As yet, 
however, good illustrations are few. 

HINGE OR PIVOTAL FAULTS 

A common type of faulting is displacement about an axis normal 
to the fault plane, one part of the block going up and the other 
part going down. (See Fig. 23.) If the fault plane is inclined, 
pivotal faulting may give an apparent normal fault on one side of 
the axis and an apparent reverse fault on the other side. Faults 
of this kind are numerous in the areas of surface volcanics in the 
West. (See map of the Iron Springs District, Utah, Bull. 338, 
U. S. G. S.) 

1 Gilbert, G. K., Bull. 324, U. S. Geol. Survey, 1907, p. 4. 



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FOLDED FAULTS 51 

CURVED AND FOLDED FAULTS 

Curved fault and joint surfaces, especially joint surfaces, may 
be formed by spalling of surfaces, caused by insolation, and other 
processes. Fractures related to the cooling of igneous rocks may be 
curved. Curved fractures are found in other relations where it 
is not easy to analyze causes, though there is no reason to doubt 
that they are governed by principles already described. 

After fracture planes are formed, they may be faulted or folded. 
Folded thrust fault planes are described and figured by Keith in 
the Roan Mountain folio of the southern Appalachians * (Figs. 30, 
31 and 32), and by Richards and Mansfield 2 in the Bannock over- 
thrust in southeastern Idaho. When previously fractured rocks 
undergo conditions of flowage, the fractures are obliterated. 

Folded fault planes should not be confused with the curving 
of fault lines on the erosion surface, due to irregularities of topog- 
raphy. 

FAULTS PASSING INTO FOLDS OR INTO SCHISTOSE ZONES 

A fault may pass into a fold along the strike, down the dip, or 
even up the dip. The intimate relation of thrust faults and over- 
thrust folds has already been cited for the southern Appalachians. 
Cadell in his experimental work illustrating the faults of the 
Scottish Highlands showed that the displacement by faulting 
below might take place above by folding. 3 The Kaibab fault of 
the high plateaus of Utah, a normal fault, grades along the strike 
into a monocline. 

Below the surface fractures die out, at depths varying with the 
strength of rocks, when the zone of flowage for these given rocks is 
reached. Displacement may be accomplished by rock fracture 
above and by rock flowage below. If a cube of soft clay be com- 
pressed from one side, held stationary at the ends, and with room 
for escape upward, a thrust fault will be developed on its upper 
side dipping toward the thrust. Lower in the cube this thrust 

x Geol. Atlas U. S., Roan Mountain folio, No. 151, U. S. Geol. Survey, 1907. 

2 Richards, R. W., and Mansfield, G. R., The Bannock overthrust; a major fault 
in southeastern Idaho and northeastern Utah: Jour. Geol., Vol. 20, 1912, pp. 681- 
709. 

8 Cadell, H. M., Geological Structure of northwest Highlands of Scotland: Mem. 
Geol. Survey of Great Britain, 1907, pp. 473-476. 



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52 



STRUCTURAL GEOLOGY 




Fig. 30. Map of the faults in the Roan Mountain and adjacent quadrangles, 
Tennessee and North Carolina, showing the relation of the minor faults 
(lighter lines) to the earlier major overthrust (heavy line). Curved fault traces 
result from folding and unequal erosion. After Keith. 




Fig. 31. Theoretical section across Buffalo Mountain and Limestone Cove, Ten- 
nessee. After Keith. Shows the character of the deformation and the rela- 
tion of the younger faults to the older overthrust. Major overthrust, heavy 
continuous line; minor faults, broken heavy line; Oa, Athens shale and over- 
lying beds; COk, Knox dolomite; CI, Cambrian limestones and shales; Cq, 
Cambrian quartzites and slates; ^lg, Archean granite and gneiss. 




Fig. 32. Theoretical section showing supposed relations of beds in Fig. 31 after the 
major faulting but before the later folding and faulting. After Keith. 






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CORRELATION OF FAULTS 53 

fault will grade into a deformation which approximates rock 
flowage. 

Chamberlin 1 has suggested that certain great thrust planes, 
such, for instance, as the one described by Willis in the Front 
Range of the Rockies in Montana 2 may be the equivalent of 
deformation by flowage down the dip of the fault plane. Van Hise 
and Chamberlin have both regarded as probable the slipping of an 
outer brittle and competent zone of fracture over a lower zone of 
flowage by tangential shearing in the upper part of the zone of 
flowage. Chamberlin would regard thrust faults as merely the 
surface manifestations of this deep-seated shearing. 

CORRELATION OF FAULTS 

The complexity of fault phenomena makes it difficult to dis- 
cover true causes or displacements. For the same reason, it is 
hardly legitimate to infer extensions or correlation of faults be- 
tween separated areas. In only a few districts are the fault direc- 
tions sufficiently uniform to warrant their correlation with faults 
of substantially the same directions in other districts. Especially 
is the extension and correlation of faults unwarranted in regions 
of igneous rocks where, as shown by the various maps of western 
mining districts (such, for instance, as the Tonopah, Clifton, 
Globe, and Bisbee) faults run in nearly all directions, intersect at 
all angles, change their directions, are cut off suddenly, and in 
fact, show all the irregularities to be expected from interior strains 
of intrusion and cooling. One is scarcely warranted in one of these 
camps in extending a fault on the map ten feet beyond where 
definite evidence of it is seen, for it may suddenly end or change its 
direction entirely. Scarcely less irregular are the joints caused by 
drying and settling of sediments. When one considers the hetero- 
geneity of rocks taken on a large scale, it is to be expected that even 
though the stresses are applied in a uniform direction over a large 
area, these stresses will be carried and resolved in such directions 
and intensities as to develop fractures with great variety of atti- 
tudes. Hence the difficulty of correlating faults over wide areas or 

1 Chamberlin, T. C, The fault problem: Econ. Geol., Vol. 2, 1907, pp. 585-601; 
704-724. 

2 Willis, Bailey, Stratigraphy and structure, Lewis and Livingston ranges, 
Montana: Bull. Geol. Soc. Amer., Vol. 13, 1902, pp. 331-336. 



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54 STRUCTURAL GEOLOGY 

between heterogeneous systems of rocks can scarcely be over- 
estimated. 

Moreover, after rocks have been fractured they may be de- 
formed by folding, in which case the fault and joint planes may be 
so distorted that they will appear on the surface as curved lines. 
The folded thrust fault planes in the southern Appalachians 
illustrate the remarkable complexity which may be developed in a 
joint or fault plane. Topographic irregularities cause a fault plane 
with low dip to appear curved on the surface. The surface distri- 
bution of such folded faults has little similarity to the idealized 
sets of straight line intersecting faults often presented as typical 
of fault conditions. 

RELATIVE NUMBER OF NORMAL AND REVERSE FAULTS 

The prevailing impression is that normal faults are more com- 
mon than reverse or thrust faults, as indicated by the use of the 
term "normal." Chamberlin and Salisbury estimate that prob- 
ably 90% of the known faults are normal. 1 A compilation made 
from all the faults indicated in the cross sections of U. S. Geological 
Survey folios fails to show such large dominance of normal faults. 
Whatever the true relative abundance, it should be kept in mind 
that this comparison only covers cases of apparent displacement 
in a vertical plane. It is likely that faults with nearly horizontal 
displacement are much more abundant than has been supposed. 

A subject for inquiry is suggested in the relative abundance 
of normal and thrust faults in rocks which have been deformed 
only at the surface, as compared with rocks which have been de- 
formed deep below the surface and subsequently exposed by ero- 
sion. Casual inspection of the available data, particularly the 
frequent association of thrust faults with phenomena of the zone 
of rock flowage, suggests that thrust faults are more common in 
rocks which have been deformed deep below the surface, while 
normal faults seem to be characteristic of surface deformation. 
Normal faults may imply extension of area which is possibly only 
at the surface. Of course there can be no clean-cut discrimination 
of two zones. When thrusts are exposed by erosion they may 
have superposed on them normal faults characteristic of surface 
conditions. 

1 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. 1, p. 498. 



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LENGTHENING AND SHORTENING OF FAULTS 55 

RELATIVE SHORTENING AND ELONGATION OF THE EARTH'S 
CRUST BY FAULTING 

Detailed studies of actual fault displacements are so few and far 
between that little can be said as to the actual elongation or short- 
ening of large parts of the faulted earth's crust. Until this is done, 
it is perhaps premature to consider general questions like the 
shortening or elongation of the earth's crust in a faulted area. 
Attempts have been made which suggest some of the following 
tentative and rather vague considerations. 

The displacements in normal faults may be assumed to be 
dominantly radial with regard to the globe, and as the dip of 
the fault plane is seldom exactly vertical, the downward move- 
ment requires extension of the horizontal surface. Compression 
faults may be supposed in general to represent tangential shorten- 
ing, with subsidiary vertical displacement. 

In view of the difficulties, already cited, of determining locally 
whether a fault represents tension or compression in three dimen- 
sions, it is obviously impossible yet to answer the question for 
large areas as to the quantitative effect of faulting on the extension 
or shortening of the earth's surface. For a given area tension 
faults at the surface may be much more numerous than thrust 
faults, yet the lengthening of the surface represented by the tension 
faults may be of less amount than the shortening of the surface by a 
thrust fault. The dip of a thrust fault plane is usually low, that of 
a normal fault plane, high. An average from the United States 
Geological Survey folios gives a dip of 36° for reverse fault planes 
and 78° for normal fault planes. A displacement of a foot on the 
thrust fault plane means nearly a foot of horizontal shortening; a 
displacement of a foot on the normal fault plane means but a few 
inches of horizontal lengthening. A single thrust plane of low dip, 
then, may accomplish a horizontal shortening which would require 
for compensation a large number of normal faults. 

If the crust as a whole has been shortened by mountain folds, it 
might appear that thrust faults are probably the dominant struc- 
ture, and that all tension faults are ultimately subsidiary phenom- 
ena. 

Geologic history seems to point to alternations of great compres- 
sive and relaxational movements. During a period of mountain- 
making, compressive stresses develop, resulting in tangential 



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56 STRUCTURAL GEOLOGY 

deformation in a comparatively short space of time, with sub- 
sidiary radial deformation in areas of uplift. During the succeed- 
ing period of quiescence it may be supposed that the action of 
gravity on uplifted areas may develop normal faults which par- 
tially compensate for the earlier shortening. 

The extension of areas caused by normal faults due to the cooling 
of igneous rocks or the drying and settling of sediments is com- 
mensurate with the shrinkage of these rocks during these processes; 
such faults cause no real extension of the earth's surface. 

There have been some attempts to calculate the lengthening or 
shortening of an area on the assumption that the displacements 
shown in cross sections are the real displacements, without taking 
into account the probability of displacement in the third dimen- 
sion. One of the few attempts to consider the problems in three 
dimensions is that of Emmons and Garrey who have estimated the 
actual extension by faulting of the Bullfrog district of Nevada. 1 
They show that the apparent extensions in individual cross sec- 
tions are greater than the real extensions because there has been 
much movement in directions inclined to the plane of the cross 
section shown by striations on fault surfaces. From somewhat 
careful quantitative study they conclude that the apparent exten- 
sion should be reduced by at least one-third to approximate the 
real extension of the area. 

EVIDENCE OF FAULTING 

The existence of faults may be determined by: 

1. Fault scarps, where the faults are recent, and erosion has not 
had time to reduce them. Excellent examples are the Hurricane 
fault scarp of the Wasatch front and the scarps so conspicuous in 
the Basin Ranges. 

2. Linear features in the topography may be caused by faulting 
which brings into juxtaposition rocks of differing resistance to 
erosion. 

3. Areal distribution of rocks or of erosion forms follows certain 
general laws, the variation from which requires the consideration 
of faulting as a disturbing factor. Illustrations may be found in 
any faulted district. 

1 Ransome, F. L., Emmons, W. H., and Garrey, G. H., Geology and ore deposits 
of the Bullfrog district, Nevada: Bull. 407, U. S. Geol. Survey, 1910, p. 88. 



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EVIDENCE OF FAULTING 57 

4. Erosion may develop drainage lines on fault planes. This 
and the other fault evidences above mentioned are further dis- 
cussed on a later page under the heading "Surface expression of 
Faults." 

5. Faulting is usually accompanied by a shear zone or the divi- 
sion of the rock into slices parallel to the plane of the fault. 

6. Faulting may be accompanied by brecciation. 

7. Faulting may be accompanied by the grinding up of the 
rock into a clay-like mass, ordinarily called " gouge." Fault 
gouge is some times really clay; more often, however, it is the 
ground up rock from which the bases have not been removed. 

8. Striations on fracture surfaces of course suggest faulting. 

9. Displacements of dikes and veins give some of the most 
easily recognizable evidence of faulting. 

It is seldom that any one of the above criteria will be entirely 
decisive in itself. Particularly is it true that an apparent fault 
scarp should not be accepted as conclusive proof of faulting until 
faulting has been otherwise substantiated. Still less is it true that 
drainage lines can be accepted in themselves as evidence of faulting. 
Even gouge, breccias, etc., may be developed under conditions 
other than faulting. 

SURFACE EXPRESSION OF FAULTS 

Normal faults may find expression at the surface in escarp- 
ments, fault traces, drainage lines, or modified distribution of 
the rocks. Escarpments may appear where the displacement is 
recent and erosion has not had sufficient opportunity to reduce 
the inequalities or where the deformation has brought into juxta- 
position rocks of differing hardness, thus permitting inequality 
of erosion on the two sides of the fault plane. In this case the 
downthrow side of the scarp may or may not be the downthrow side 
of the fault. 

Among the best known instances of faults still represented by 
the original escarpments are the Hurricane fault separating the 
Wasatch Mountains from the Great Basin, and the faults of the 
Great Basin ranges, which were originally classified by Gilbert 1 

1 Gilbert, G. K., Report on the geology of portions of Nevada, Utah, California, 
and Arizona, examined in the years 1871 and 1872: U. S. Geog. Surveys W. 100th 
Mer., Vol. 3, 1875, pp. 17-187. 



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58 



STRUCTURAL GEOLOGY 



as an example of the block type of mountains. A fault scarp 
resulting from recent displacement accompanied by earthquakes 
in Alaska is illustrated in Fig. 33. The criteria used by Gilbert 
for the recognition and delineation of fault scarps of the Great 
Basin are (a) their steepness, (b) their association with shear zones 
and displacement of beds, (c) displacement of plateau level as in 
the Hurricane fault of Utah, (d) the fact that the scarps may not 
converge toward the end of the mountains as they would if they 









■ 



Fig. 33. Fault scarp developed during earthquake of 1899, Yakatut Bay region of 
Alaska. After Martin. 

were normal erosion scarps, (e) the existence of triangular facets 
across the ends of ridges as though the ridge had been sliced off, 
(f) the recent displacement of alluvial fans, lake beds, and other 
surface features, indicating that the faulting has been going on to 
very recent times and has not had time to be masked by erosion. 
Spurr 1 questions these criteria for the surface delineation of faults, 
or rather, the degree of emphasis to be placed on them. He calls 
attention to the fact that erosion has been conspicuously effective 
in producing the present topographic features of some of the Great 
Basin ranges, that anticlines and synclines play an important 

1 Spurr, J. E., Origin and structure of the Basin Ranges: Bull. Geol. Soc. Am., 
Vol. 12, 1901, pp. 264-266. 



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SURFACE EXPRESSION OF FAULTS 59 

part, and that the recognized faults in these ranges are often 
quite independent of the topographic features. The student 
may study maps of these ranges to advantage, keeping in mind the 
criteria above cited. On these and on the Terlingua, Texas, 
topographic map, specific topographic features seem to indicate 
recent faulting. 

In districts of older deformation, like the southern Appala- 
chians, erosion has had a longer time to develop the topographic 
features, with the result that original fault scarps are practically 
non-existent. The effect of faults on the topography is due to their 
bringing into contact rocks of unequal hardness, thus permitting 
differential erosion. The flat and curved attitudes of the fault 
planes here also tend to make them less conspicuous in the topog- 
raphy. 

Thrust faults are not likely to produce steep vertical escarp- 
ments, either before or after erosion. By pushing forward succes- 
sive slices of rock they tend to cause linear features in the topog- 
raphy, and yet these features are not different from those which 
might have been produced by folding and probably they would 
not be identified as related to thrust faulting unless the thrust 
faulting had been otherwise proved. 

In regions of vertical faults, especially in flat-lying beds and non- 
glaciated areas, the lines of the faults are very likely to be marked 
by drainage channels which have developed along these planes of 
weakness. All faults are not marked by drainage lines, nor do all 
drainage lines mark faults. (See p. 12.) 

One of the most fully studied cases of the surface expression of a 
fault with horizontal displacement is that of the fault which 
caused the California earthquake of 1906. "At the surface the 
cracks had great variety of expression. Some were barely percep- 
tible as partings; others gaped so widely that one might look down 
them several yards. Some were mere pullings apart; others showed 
small differential movements of the nature of faulting. Some were 
solitary; others, especially those exhibiting faulting, were in 
groups." * Where the fault crossed a spur or shoulder of a moun- 
tain a scarp appears. Small basins or ponds, many having no 
outlets and some containing saline water, are frequently found at 
the base of small scarps. Troughlike depressions appear on both 

1 Gabert, G. K., Bull. 324, U. S. G. S., 1907, p. 7. 



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00 STRUCTURAL GEOLOGY 

sides, also bounded by scarps. Small knolls or sharp little ridges 
are common at the fault line and these are bounded on one side by 
a softened scarp and separated from the normal slope of the valley 
side by a line of depression. Other effects of this fault are slides of 
earth or rock from the hillslopes. Finally, there are many con- 
spicuous dislocations of the works of men. 1 

The relations of valleys and particularly lakes to fault displace- 
ments may be studied to advantage in the northern part of the 
Santa Cruz, California, folio. 

SUGGESTIONS FOR LABORATORY STUDY OF FAULTS 
(See also page 31) 

Much can be done in the study of faults on geologic maps, referred to 
on the foregoing pages, particularly in the section on the surface expression 
of faults. It is suggested that these maps and others named below be 
studied with a view of answering specifically the following questions: 

How are the faults indicated by the topography, or the distribution of 
the outcrops? What is the dip of the fault plane? What is the apparent 
displacement? Is there any way of ascertaining the real displacement? 
Considered in three dimensions, what has been the deformation accom- 
plished by the faulting? The dip of a fault plane may be determined in 
some cases by actual measurement, in others by the relation of outcrops 
to topography. What are the possible relations of the fault plane to 
stresses producing it? 

Morristown, Tennessee, folio (No. 27) U. S. Geol. Survey. 

Roan Mountain, Tennessee-North Carolinia, folio (No. 151) U. S. 
Geol. Survey. 

Stratigraphy and structure, Lewis and Livingston Ranges, Montana, 
by Bailey Willis: Bull. Geol. Soc. Am., Vol. 13, 1902, pp. 305-352. 

Anthracite-Crested Butte, Colorado, folio (No. 9) U. S. Geol. Survey. 

Silverton, Colorado, folio (No. 120) U. S. Geol. Survey. 

Bisbee, Arizona, folio (No. 112) U. S. Geol. Survey. 

The Bannock overthrust; a major fault in southeastern Idaho and 
northeastern Utah, by R. W. Richards and G. R. Mansfield: Jour. Geol., 
Vol. 20, 1912, pp. 681-709. 

The interpretation of topographic maps, by R. D. Salisbury and W. W. 
Atwood: Prof. Paper No. 60, U. S. Geol. Survey, 1908, p. 77. 

The geological structure of the northwest Highlands of Scotland: 
Memoir Geol. Survey, Great Britain, 1907, pp. 463-476. 

Report on an investigation of the geological structure of the Alps, by 
Bailey Willis: Smithsonian Misc. Collections, Vol. 56, No. 31, 1912. 

Experiments on faulting must be limited to the equipment available in 
the laboratory. If there is available equipment for compression tests of 

1 Lawson, A. C, Preliminary report of the State Earthquake Investigation Com- 
mission, 1906, and final report, 1908. 



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LABORATORY STUDY OF FAULTS 61 

stone or building materials, this can be advantageously used in experi- 
ments of the kind referred to on page 16. 

There are other simple and inexpensive devices for showing most of the 
facts discussed on the foregoing pages. One of these is the wire screening, 
described on pp. 18-20. It is easy to devise apparatus for the deforma- 
tion of clay and small plaster of Paris blocks, because the stresses required 
are very moderate. Much can be done with these materials with an 
ordinary vise or in using clay with a box with movable sides working under 
screw compression. 

Another simple device is a rubber sheet mounted on a frame with ex- 
tension screws, so that it may be stretched or contracted in any direction. 
The sheet is coated with paraffine and stretched. Tension fractures 
develop normal to the stretching. If the rubber sheet be first extended in 
one direction and then coated with paraffine and allowed to contract, 
compression fractures develop in planes dipping toward or away from the 
compression or contraction and striking normal to the direction of move- 
ment. Contraction in one direction is accompanied by expansion in a 
direction normal to it, resulting in tension fractures normal to the com- 
pression fractures. This simple equipment also allows of experiments 
involving warping and rotational stresses. 

The deformation on the surface of an expanding or contracting rubber 
sheet is perhaps more nearly like rock deformation in nature than the 
deformation produced by applying external pressure through the sides of 
a rectangular box, for so far as the conditions can be inferred in nature, 
the stresses are ordinarily not applied externally against definite faces of 
the deformed mass, but are distributed throughout considerable rock 
masses. 

FRACTURE CLEAVAGE AND FISSILITY 

Fracture cleavage may be defined as a structure inherent in a 
rock mass whereby under stress it breaks along closely spaced 
parallel incipient joints. It differs from flow cleavage in features 
noted below. The term fissility has been used by Van Hise 1 for 
the actual parallel partings; but he uses it also to include capac- 
ity to part along such parallel planes. In the latter usage it is 
practically synonymous with fracture cleavage. It may be desir- 
able to retain the term fissility as strictly defined by Van Hise 
for the actual partings, as distinguished from fracture cleavage, 
which applies to the capacity to part. Other terms more or less 
synonymous with fracture cleavage are close-joints cleavage, "aus- 
weichungs" cleavage, fault-slip cleavage, rift, etc. (Figs. 34-37). 

Fracture cleavage is a fracture phenomenon and is developed 

1 Van Hise, C. R., Principles of North American pre-Cambrian geology: 16th 
Ann. Rept. U. S. G. S., pt. 1, 1896, p. 633. 



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62 STRUCTURAL GEOLOGY 

under the general stress-strain relations already discussed for 
joints and faults. In some cases the surfaces of weakness are 
clearly cemented joint surfaces. In other cases there is no evidence 
that there has ever been actual parting followed by cementation; 
the surfaces seem to be incipient fracture surfaces along which the 
rock is still coherent, like cracks in a plate which has not yet fallen 
apart. Arrangement of the mineral particles with their longer 
axes in the plane of fracture cleavage is not a necessary condition, 
though this arrangement is often secondarily developed by rubbing 
between the parts. Fracture cleavage may be partly the result of 




Fig. 34. Fracture cleavage developing polygonal blocks in slate previously possess- 
ing flow cleavage. 

minute relative displacements along incipient fracture planes by 
minor monoclinal folding or faulting of the distributive type men- 
tioned in another place. (See p. 48 and Fig. 35.) 

Fracture cleavage planes are more widely spaced than "flow 
cleavage" planes (see p. 76) and are characteristically in two 
or more intersecting sets, allowing the rock to break into various 
polygonal forms. In some rocks one set is so dominant and so 
closely spaced as to give a structure very closely simulating flow 
cleavage; indeed, there are many rocks in which the structure 
cannot be satisfactorily designated either as fracture or flow 
cleavage, but is in reality some combination of the two. 

Fracture cleavage, as a phenomenon of the zone of rock frac- 



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FRACTURE CLEAVAGE 



63 



ture, may be superposed upon rocks which had before been in the 
zone of rock flowage. The previous existence of a good rock cleav- 
age developed in the zone of flow favors the development, in the 




Fio. 35. Photomicrograph of slate with false or fracture cleavage from Black Hills 
of South Dakota. The longer diameters of the particles, mainly mica, quartz, 
and feldspar, lie, for the most part, in a plane intersecting the plane of the page 
and parallel to its longer sides, but in well-separated planes at right angles to 
this plane the longer diameters of the particles have been deflected into minute 
monoclinal folds represented by the darker cross lines. In these cross planes 
also porphyritic biotites have developed with their longer diameters parallel. 
The rock has two cleavages, one conditioned by the prevailing dimensional 
arrangement of the minute particles and the other conditioned by the planes of 
weakness along the axes of the minute monoclinal folds crossing the prevailing 
cleavage. The first cleavage is flow cleavage developed in normal fashion 
during rock flowage, and the second is of the nature of fracture cleavage 
developed later along separated shearing planes in the zone of fracture or in 
the zone of combined fracture and flowage. The rock cleaves into parallelo- 
piped blocks. 

zone of fracture, of closely-spaced parallel planes of parting, 
yielding fracture cleavage or fissility. 

On the other hand, if a rock with fracture cleavage comes into 
the zone of rock flowage, the structure is obliterated. 

A common example of the development of fracture cleavage 



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64 



STRUCTURAL GEOLOGY 



or fissility is found where a soft bed is deformed by fracture be- 
tween two stronger beds, as for instance, the Baraboo quartzite. 
Here curved fissures are formed by compression (see Figs. 9, 10, 
and 11), and these are crossed by tension cracks. The mechanics 
of this problem are discussed on pp.16-21. 



X 55 c//am. _ 




Fig. 36. Fracture cleavage crossing flow cleavage. After Dale. 

BRECCIAS AND AUTOCLASTICS 

When rocks are broken into irregular angular fragments they 
are called " breccias," "friction breccias," or " autoclastics" 
(" self -broken" rocks). They may be cemented by infiltration. 
Such rocks may be difficult to discriminate from conglomerates 
or " elastics" formed by the ordinary processes of erosion. Some 
of these differences are as follows: (a) The fragments in the auto- 
clastic rock are usually more angular than those of the con- 
glomerate, but to this there are exceptions, (b) They are likely to 
be more homogeneous in character; ordinarily they are of one kind 
of rock. Clastic rocks may have several kinds of fragments coming 
from different sources. However, many elastics are made up 
dominantly of one kind of fragment; hence this criterion is also 



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BRECCIAS AND AUTOCLASTICS 



65 



inconclusive, (c) The cement of an autoclastic rock is likely to be 
vein material, while that of a clastic is usually fine-grained frag- 
mental material. This is one of the safest criteria, (d) An auto- 
clastic rock may be developed in zones crossing the bedding. This 




Fig. 37. Fracture cleavage, jointing and flow cleavage developed in graywacke 
and slate, Alaska., After Gilbert (photograph by U. S. Geol. Survey.) Use 
principle of strain ellipsoid (See pp. 18-21) to ascertain direction of relative 
displacement and theoretic position of fracture planes and flow planes. (See 
also Figs. 9 and 10). 

occurrence sometimes gives a clue as to its origin. No one of the 
above distinctions is decisive. Collectively they may be so, but 
not in all cases. 



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I'Ul 

I 1 t>U4 I iU v>\ v « tV III 



.K 



\u 



v . v 



»*» snucTLKAL Mx.'LmiY 

li. *. .u ,. .«,UK-lii«ii,-K and wiMsSrxttawarft. have been rendered 
., :•. . ( >'.-v. iik- ..iiti,-uli> ot diMTiiiiiiiati.^ i» increased. \n \rchean 

; ' ' ,IK " w '*» ,1 "»> *>wtrict of Minnesota is sheared into 

" •'- ,s ' km> ,HVU «»"««*1 by fiowage, and brought 

'""* l * ^'"R^l !«•*», like pebbles. This pseudo- 

'v.HM-i-umtuiU-doiUy with the greatest difficult 

* tiiio wwRlomcrate formin* the base of the 
V L> " *' ;a u ' ' x ' Ma ° u < 1 ilwuitiantly of pebbles of porphyry, 

■d ! Mtvn iW often simulate autoclastic rocks in 
- Wt tn»m fracture, but differ from auto 

• - * m« i* caused by volcanic explosion rather 
>* m«v W acini* directly through the earth. 
* • - bounced by the cooling, hardening, and 

- - — hw nv lule still flowing, likewise resemble 
* ,:ciuirK-aiKui of tuffs and volcanic breccias 

- v ^^^^tuMtyuftheirfragments,(2)inthe 

- i-muivk <uch as amygdules peripherally 
v,Uv,u:k!ou ^ th <^ fragments by volcanic 
- v v,t^u ..art ty of the fragments. None of 

-^c u i^nmmatmg tuffs from elastics or 

" ,H,ui ;U ^ 1 NV * UT are distinguished onlv 

! — > tvm ^utW n >osited elastics resulting 

1 — ^' l -* "vi-vs associated with the immense 
> ,v — '^ --tke Superior region well illus- 

v ,v ... ^s.-:: vhomiuc rocks having amygda- 

• v .. ■, k Xl -*» ?<*< uuly when the volcanics 
^ * * "^ •* ^ uc.rv structures of limestone. 

v , v .xuxnvu u :he character of auto- 

vv ^ ^' '<- - '' M ^-^s- of rheir discrimina- 

^ •- x. ,,. t! . tr :vck^ It has- been the 

"'" ,4 ; -m — -<<- -^ t>>* - duties- have been a 

,„,»,},. v*. ...»,. m,-. *,.,,. , ^^ ,-,.; -t^tusiou. especially in 
.rhs.tv.^ uv,.-.. i ,^v , V .- Ttl> -^, v , tl tie <upertieial aspect 

,»i ■ tio TtnA v.-.w \ ** •■. ,-. x^ *-..frra ^loma "leappiiedin 

,ici:ui uiu ttcvttv v:x-r w ^, v fc . ^>^--e -^.uogic evidence. 



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I t« M« ' tl 




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"**- v ~ : "**- -LT^ -..•*— — -** ~^r"\_ . i^it* Ti"^ zricn z5fr?n* n- 

*▼■—*";. *> Tr "^r"..-^ =■ — . *r~i "^♦^m mr.ti. " ~ ?:ir ~w^zas*± r:«*£s 

.r^- ~.t-:j '* m -z-r -^r^~ \_«r -^TM .: :::.»v- T~i-» -n:*y ~r ^v<r tie 
•?r- — -~ •*" " : ~'_j~ "-^Zi "> ilir -^rn.:i ^:rrH « 1:^7- m 2L±zy rsLTTS 
»i ..- ^ •' v tt =-* ~_- n~"- '»*-"irTHi£: jl "tune's ay-^nrrTi'ri :ytrje 
r^".^* -^^n. r n 1 -u^^a ulil ~~i #+*\t -sio*£- m ;; times i^er- 
x-nr-: " • .r- ur*-r. a r ~:e *^r~JL*:"2L£i£ vi-^ >: fir is ibe 
t— -r iiii -v- - .--- i^* - nn:^-in—-7 5^r »tis*es :r* Vc: systems 

- mz.'^-.i -Lx : -^^-. liui 11 rr — ls"-lzi ±zj£ izj ^m 2£SS»;<i^S: ns *;f 
■*ir r :«: l^-x.— ^ — : 1 :t.:.:e ram* c i* :.' — ~^ir ^^t^oSl Ir?5rpe-:>i- 
**i" r r*i" *-^. >\"Zi *:iii" ^--ltmc •ji: ^ r:v T -.f.Tv- :c j-:cmirg. it 

z* >:v" ;r rj!T» 21 >r ri^l "*~tjlzl "U^ rizii^ :c liie ^znirhis cases of 
~-i~» a t^Lt- "ci:r^-u c. i_rf^iy 5r-si!r:*»ei- S> fsr ss the earth- 
v^ix-* ser-17 ii— ^l~~i~^ *^jt <^rijz- i^rr^iy present, h is obvious 
"i*i3 v_h- -~-l-Z- nn 7 "*r rir_er ~J>e re^in of lensSon or compression. 
'*•; :u is 1 "iir ^-ri'. -jJl-t TiTr hseif drrr-k»ps ibe strain- it seems 
. ^'7 !--♦; *:^: '• "1 ••er^oic-al ai»d compressive joints might be 
*-"'** ~*~\. i_"i- *-jn i. •* ^il j»r^»f of one or the other is difficult to 
'.">- Lr:,^:^ wive* ir^v vibrate pjarallel or normal to the 
' : . r**r YX+ <£ triii-mi— i<:»n. TL<:»^ vil trating piarallel to the direction 
of triTviiii^k*n may cause bc»th ccHnpressi<Mi and tensicMi. The 
qi'-Tion difficult to answer is, which of these waves, or which 
'ornporj^nt of the^e waves, first sxnpasses the breaking strength of 
the ror-k? In general rocks may be expected to yield to tension 
first. 

The effects of earthquakes on building and other structures need 
not be detailed from our point of view, because they do not con- 
stitute a part of the geological record under consideration. They 
are of interest from a geological standpoint as showing the direction 
of transmission and vibration of earthquake waves, (see page 74) 
the displacements along faults, etc. The maximum shaking and 
destructive effects of earthquakes appear to be in loosely con- 
solidated rocks, gravels, and soils which are saturated with water. 
The reason for this is not entirely clear. It has been suggested 
that the water affords opportunity for the materials to move 



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68 STRUCTURAL GEOLOGY 

of the wave by the tensional component. Intersecting sets there- 
fore would require successive earthquake waves from different di- 
rections. He further has shown experimentally that where rocks 
are already under strain the earthquake wave may bring the 
stresses beyond the breaking point simultaneously in many parts 
of the rock mass, the joints occurring in planes determined by the 
initial strain, or, in a sudden and violent shock, in planes deter- 
mined by the direction of the earthquake wave. So far as the 
writer knows, there are comparatively few cases of joint systems 
which can be definitely proved to be related to earthquakes, 
notwithstanding the inherent probability that earthquakes ac- 
complish such results, and notwithstanding known associations of 
earthquakes with a plane or zone of faulting (see p. 58.) Independ- 
ent of any real bearing that earthquakes may have on jointing, it 
is to be remembered that the actual stress-strain relations at the 
point of rupture must fall within the range of the limiting cases of 
tension and compression already described. So far as the earth- 
quake merely accentuates the strain already present, it is obvious 
that the strain may be either the result of tension or compression. 
So far as the earthquake wave itself develops the strain, it seems 
likely also that both tensional and compressive joints might be 
expected, although actual proof of one or the other is difficult to 
cite. Earthquake waves may vibrate parallel or normal to the 
direction of transmission. Those vibrating parallel to the direction 
of transmission may cause both compression and tension. The 
question difficult to answer is, which of these waves, or which 
component of these waves, first surpasses the breaking strength of 
the rock? In general rocks may be expected to yield to tension 
first. 

The effects of earthquakes on building and other structures need 
not be detailed from our point of view, because they do not con- 
stitute a part of the geological record under consideration. They 
are of interest from a geological standpoint as showing the direction 
of transmission and vibration of earthquake waves, (see page 74) 
the displacements along faults, etc. The maximum shaking and 
destructive effects of earthquakes appear to be in loosely con- 
solidated rocks, gravels, and soils which are saturated with water. 
The reason for this is not entirely clear. It has been suggested 
that the water affords opportunity for the materials to move 



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EARTHQUAKES 69 

easily, and by filling all the pore spaces that it aids in the trans- 
mission of the shock. 

Observations taken in sounding and on the breaks in cables 
following earthquakes have shown that large segments of the 
bottom of the ocean have dropped hundreds of feet as a result or 
cause of such shocks. Ordinarily, the accompanying continental 
changes have been of smaller magnitude and usually uplifts 
rather than depressions. Continental changes, while considerable, 
are, as listed by Milne, commonly measured by units of a few feet or 
a few tens of feet. 

Within our zone of observation earthquakes are clearly re- 
lated to rock fracturing, but it is not certain that they may not 
also have relation to sudden deformation by rock flowage at points 
below our observation. When it is remembered how intimate is the 
association of fracturing and rock flowage, how the two processes 
seem in some places to go on side by side under the same stresses, 
it becomes obviously difficult to exclude rock flowage from con- 
sideration in connection with earthquakes. 

Rock flowage as a result of earthquake shock is even more prob- 
able. If local stresses are almost of the necessary magnitude to 
produce rock flowage, it is conceivable that the earthquake shock 
may carry these stresses past the resistance of the rock and require 
rock flowage. Also, it may be supposed that rock flowage already 
started may be accelerated by earthquake shocks. 

KINDS OF FRACTURING ACCOMPANYING EARTHQUAKES 

It is not easy to correlate earthquake shocks with particular 
kinds of fractures. It is the natural assumption that faults with 
great displacements cause great earthquakes, yet so far as histori- 
cal records go, great earthquakes have sometimes been associated 
with apparently slight breaks. There is nothing yet to show that 
earthquakes are associated alone with compressive or with tension 
fractures. So far as field observations inform us, earthquakes are 
most frequently related to vertical fissures, but certainly one must 
suppose that great thrust faults initiate earthquakes. The fact 
that so many earthquakes can not be definitely connected with 
fractures at the surface may indicate their relation to thrust faults 
forming beneath the surface. Milne regards the minor earth- 



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70 STRUCTURAL GEOLOGY 

shakers or "microseisms" as the result of minor settlings necr 
the surface, along previously formed vertical fissures. 1 

EARTHQUAKES AND OSCILLATIONS OF GLACIERS 

Glacial oscillations have been shown recently to be related to 
earthquake shocks, 2 making it possible to interpret certain re- 
markable advances of glaciers otherwise inexplicable. 

EARTHQUAKES AND VULCANISM 

There is a relationship between earthquakes and vulcanism, both 
in time and place. Vulcanism has been accompanied in many 
places by earthquakes, and vice versa. However, there are many 
cases of earthquakes not associated with vulcanism, and of many 
volcanic outbreaks not accompanied by earthquakes. Since 
vulcanism is now generally regarded as involving mechanical 
disturbances of the crust, lessening the pressure upon the hot rock, 
and thereby allowing it to liquefy, it may be reasoned that earth- 
quakes, by disturbing the equilibrium of pressures, may be a local 
cause of vulcanism. Or, both may result from larger earth move- 
ments. 

EARTHQUAKES AND MAGNETIC DISTURBANCES 

Earthquakes are often, though not always, accompanied by 
magnetic disturbances. There are still differences of opinion as 
to whether or not these magnetic disturbances are mere incidents 
of the mechanical readjustments. There is some evidence that the 
magnetic disturbance is not entirely related to mechanical changes, 
suggesting the possibility of some further, and as yet unknown, 
underlying relationship. 

Another interesting relationship, as yet unexplained, is observed 
between some earthquake zones and regions of permanently steep 
magnetic gradients of the earth's magnetic field. 

EARTHQUAKES AND ROCK DENSITY 

Earthquakes are likely to be numerous in regions which show 
steep rock density gradients, that is, regions in which light and 
heavy earth masses are in close contiguity. 

1 Milne, John, Seismological observations and earth physics: Geographical Jour. 
Vol. 21, 1903, page 21. 

2 Tarr, R. S. t and Martin, Lawrence, The earthquakes at Yakutat Bay in Septem- 
ber, 1899: Prof. Paper No. 69, U. S. Geol. Survey, 1912. 



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EARTHQUAKES 71 

EARTHQUAKE ZONES 

The distribution of earthquakes corresponds with zones of more 
or less intense deformation or vulcanism or both. In a very 
general way there are two great earthquake zones, the so-called 
Mediterranean zone or belt passing through the Himalayas and 
eastern China, from which have started 53% of the recorded earth- 
quakes; and the Pacific belt bordering the Pacific basin, in which 
have originated 41% of the recorded earthquakes. 1 More specif- 
ically, earthquakes are likely to follow along the margins of 
continents or of smaller areas of great relief, along mountain chains, 
especially of recent origin, along volcanic belts, along margins of 
two areas differing considerably in density, as for instance in the 
zone of the Messina earthquake, and along areas where there are 
great irregularities in distribution of the earth's magnetism. It 
has been ascertained that earthquakes have been especially nu- 
merous in the geosynclines of Mesozoic rocks. As many of these 
rocks have been folded into mountain ranges in comparative^ late 
geological time, this is only a specific case of the abundance of 
earthquakes in mountains of recent origin. 

INSTRUMENTS FOR DETERMINING AND MEASURING EARTH- 
QUAKES 

Seismographs are instruments for the detection and measuring 
of earthquakes. They are made in a variety of forms but are all 
essentially devices for determining more or less independently the 
three principal components of a wave, that is, the vibrations in 
three mutually perpendicular planes. A pendulum makes an auto- 
matic record, mechanically or photographically, on a sheet moving 
at a uniform rate beneath it. The record is a straight line until it is 
disturbed by an earthquake wave, when the line becomes crenu- 
lated. The earthquake wave is expressed in the amplitude and 
spacing of the crenulations. It is more correct to say that the 
wave is expressed partly on any one record, for only those com- 
ponents of the wave that are normal to the plane of the pendulum 
are expressed. In order to get a complete record there must be 
three seismographs oriented in mutually perpendicular directions. 

1 Montessus de Ballore, F. de, Les Tremblements de Terre, Paris, 1906. 



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72 STRUCTURAL GEOLOGY 

EARTHQUAKE WAVES 

Earthquake waves are supposed to be (1) compressive or longi- 
tudinal, vibrating parallel to the direction of transmission of the 
shock, and (2) transverse, vibrating normal to the direction of 
transmission of the shock. Earthquake waves reach a distant 
point on the earth's surface both by passing along the chord and by 
going around the circumference, arriving at different times. The 
circumferential wave travels about one-third as fast as the chord 
wave. The chord wave travels through the earth's diameter in 22 
minutes. The chord waves are regarded as compressive, the cir- 
cumference waves as transverse. Milne regards it as uncertain 
whether the circumferential wave is undulatory in vertical dimen- 
sion, like a wave propelled in water, involving tilting of the surface, 
or whether it is distinctly a horizontal shaking. The Kingston 
earthquake sent out two principal shocks. The first of these, 
the one traveling along the chord, was registered on a seismograph 
at Washington (almost due north), and principally on the pendu- 
lum which vibrated east and west; the wave therefore was vibrat- 
ing north and south; it was a compressive wave. The second 
one, traveling along the circumference of the earth, was registered 
principally on the pendulum vibrating north and south; the wave 
was vibrating east and west; it was a transvere wave. This is the 
kind of evidence upon which the directions of earthquake vibra- 
tions are determined. It is not always regarded as conclusive. 

A third wave may follow at an interval which suggests that it has 
gone around the longer arc of the earth's circumference. There is 
evidence of convergence of waves at antipodal points, and of their 
resurgence to points of observation. 

CONDITION OF EARTH'S INTERIOR AS INFERRED FROM EARTH- 
QUAKE WAVES 

If earthquake waves have both circumferential and chord paths, 
their behavior points to certain differences in the physical condition 
of the media they traverse. Within 600 miles of their origin, the 
first and second waves are confused. It may be calculated that 
neither of these waves would get below 12 miles from the surface 
in traveling to a point within 600 miles. Beyond 600 miles the two 
waves become sharply separated in time, suggesting that the chord 



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EARTHQUAKES 73 

wave, which now passes more than 12 miles from the surface, is in a 
different kind of medium. It is possible that in the first instance 
the waves were passing entirely through the zone of rock fracture; 
in the second instance the deeper waves were passing partly 
through the zone of rock flow. Milne also notes that chord waves 
registered at antipodal stations and therefore traveling along a 
path nearly through the center of the earth, behave differently 
from waves passing through intermediate depths or along the sur- 
face. The former are much dampened and confused, suggesting 
still a different physical state at the center. In general, however, 
the deep or chord waves travel with such a speed as to indicate a 
rigidity nearly twice that of steel, and their uniform speed argues 
for homogeneity of the medium traversed. 1 

The evidence bearing on the nature and paths of earthquake 
waves is complex, and agreement has not been reached among 
seismologists. Some of the principal conclusions of seismologists 
on the subject have been merely noted. 

LOCATION OF THE ORIGIN OF EARTHQUAKES 

So far as earthquake shock results mainly from rock fracture, 
its origin is in the zone of rock fracture and hence probably not far 
beneath the surface. Doubtless there are readjustments in the 
zone of flow at the same time, but these may be subsidiary as 
causes. A shallow depth for the origin of earthquakes has been 
found wherever it has been possible to determine the directions of 
emergence of earthquake waves, either from instrumental observa- 
tions or from the study of the destructive results of earthquake 
shocks. Nowhere have these determinations indicated a depth of 
origin greater than 12 miles, and usually less. The very fact that 
an earthquake shock is usually so well localized at some spot on the 
earth's surface, that there is some one zone which may be regarded 
as the locus of activity, is evidence that its origin is not far below 
the surface. These observations have already been cited as 
evidence that the zone of rock fracture is not deep. Granting that 
earthquakes originate in the zone of fracture, it may be argued 
that their depth of origin at the maximum is about 10 or 12 miles, 

1 Milne, John, Seismological observations and earth physics: Geographical Jour., 
Vol. 21, 1903, p. 7. 



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74 STRUCTURAL GEOLOGY 

since there is some evidence that the zone of fracture does not 
generally extend beyond that depth. 

In the area most affected by the quake, the origin is located by 
the intensity of the shock and by noting the direction of emergence 
of the waves. The area most affected is usually roughly oval 
or elliptical and within it there is usually a line or spot at which 
the intensity of the shock is clearly at a maximum. It is as- 
sumed that near at hand the waves are both transverse and com- 
pressive, that both shearing and tensional stresses are set up in the 
structures affected, and that the breaking strength is first sur- 
passed by tensional stresses, the dominant one of which would be 
normal to the direction of transmission of the wave. Hence 
fracture planes in buildings are regarded as due to tension, and 
therefore normal to the path of the wave. The plane of fracture is 
best determined at the corner of a building. Lines drawn normal 
to these fracture planes in widely distributed areas may tend to 
converge in a point, or plane, which are then regarded as the origin 
of the quake. This method is of doubtful value, because the 
attitude of fractures is so influenced by local conditions, and it is 
difficult to prove that they are tensional. 

The location of the earthquake from more distant points is 
accomplished mainly by noting the difference in time of the receipt 
of the principal shocks, chord and circumferential. The greater 
the difference in time between the receipt of the two the greater the 
distance from the point of origin. At any one point the distance, 
not direction, is determined. It needs observations of distance 
from three points to determine, by the intersection method, the 
locus of origin. 

PREDICTION OF EARTHQUAKES 

It has not been possible thus far to predict with any considerable 
degree of success the time and place of earthquakes. As to place, 
there is the probability that earthquakes will be confined to certain 
broad zones in which they have commonly originated in the past. 
Within an earthquake zone the records seem to show that a great 
disturbance at one locality may mean that the next disturbance is 
to be looked for in some other part of the great belt. There have 
been too many exceptions to this, however, to establish the rule. 
When one notes the widespread distribution of faults over the 



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EARTHQUAKES 75 

earth's surface, most of them doubtless accompanied in their 
formation by earthquakes, and considers the possibilities for 
faulting in the geologic future, predictions as to the localization of 
earthquakes, based on the meagre records of historical time, 
can not be accepted with any great confidence. 

Attempts to establish a principle of periodicity for earthquakes 
have been equally futile. Gilbert * calls attention to the fact that 
many attempts at working out the periodicity of earthquakes are 
apparently successful because the great frequency of earthquakes 
of some magnitude furnishes examples for any time system postu- 
lated. 

1 Gilbert, G. K., Earthquake forecasts: Science, Vol. 29, 1909, pp. 121-138. 



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ROCK FLOWAGE 

A rock is said to have flowed when it is deformed without con- 
spicuous fracture, remaining at the end of the deformation an 
integral body. This interpretation does not exclude minor frac- 
tures in the constituent minerals during rock flowage. Rock 
flowage produces hard and crystalline types. The process is 
essentially a constructive and integrating one. As here used, it 
has no necessary relation to fusion, though it is possible that 
the high pressures involved may cause minerals to melt at com- 
paratively low temperatures. 1 

One of the conspicuous results of rock flowage is a slaty or 
schistose or gneissic structure, giving the rock a cleavage. All 
such structures are described below under the heading of "Flow 
Cleavage." In so far as gneissic structure shows banding, without 
cleavage, as it sometimes does, this is discussed under another 
head (p. 87). Some rocks flow without taking on either a schistose 
or slaty or gneissic structure. These are likewise discussed under a 
subsequent heading. Fracture cleavage or fissility, already dis- 
cussed, is a phenomenon of rock fracture rather than of rock flow, 

FLOW CLEAVAGE 2 

Flow cleavage is a capacity of some rocks to part along parallel 
surfaces, not necessarily planes. These surfaces are determined by 
the parallel dimensional arrangement of the mineral constituents, 
that is, by the mutual parallelism of the greatest, mean, and least 
dimensional axes of the mineral particles making up the rock mass. 
They may also be determined by the parallelism of the mineral 
cleavages of the constituent particles. 

A few minerals, such as mica, hornblende, quartz, and feldspar, in 
various ratios, make up all but a very small percentage of schistose 
or cleavable rocks. To make the discussion concrete, therefore, 
cleavage will be discussed principally in relation to these four 

1 Johnston, John, and Adams, L. H. , On the effect of high pressures on the 
physical and chemical behavior of solids: Am. Jour. Sci., vol. 35, 1913, pp. 
205-253. 

2 For fuller discussion see: Leith, C. K., Rock cleavage: Bull. 239, U. S. Geol. 
Survey, 1905, pp. 23-118. 

76 



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FLOW CLEAVAGE 77 

minerals. The technical reader will at once think of qualifications 
and additions necessary where other minerals are considered, but 
in the writer's judgment these do not essentially affect conclusions 
based on the study of a few of the principal schist-forming minerals. 

One of the peculiar features of a cleavable rock is the uniformity 
in shape of the grains of each of the characteristic minerals, deter- 
mined by their crystal habit. The average ratio of the greatest 
to the mean dimensions of a mica plate is about 10:1, of horn- 
blende 4:1, and of quartz and feldspar 1.5:1. These ratios are the 
same whether the rock cleavage is good or poor. In other words, 
the better rock cleavage does not necessarily mean a greater 
drawing out or elongation of mineral particles. 

When in the laboratory crystals are allowed to develop under 
stress, they elongate in the plane of easiest relief, supposedly, 
regardless of habit, but this is not certain, because the experiments 
have been conducted principally with isometric crystals. 1 Also, 
crystals not under conditions of growth have been elongated by 
pressure alone, again more or less regardless of habit. But not- 
withstanding these experimental results, the minerals in schists 
have an elongation ordinarily determined by habit alone. The 
difference between a schist with poor cleavage and one with good 
cleavage is not so much that the particles of one have been elon- 
gated more than the particles of the other, but that it has more of 
the kinds of particles which by habit are elongated. 

There is, in the schists, relative perfection of crystal forms, 
dependent on the character of the minerals, as compared with 
igneous rocks, where shape of the minerals depends more largely 
on order of crystallization. This mineral form and arrangement in 
schists is the " crystalloblastic " structure of Milch 2 and Gruben- 
mann. 3 

The parallel dimensional arrangement of the mica and horn- 
blende, and sometimes the feldspar, implies a parallelism of their 
mineral cleavages, because these minerals tend to occur with 
definite crystal habit within the rock, and the mineral cleavages 
are definitely oriented with reference to the dimensional axes. 

1 Becker, G. F., and Day, A. L., Linear force of growing crystals: Proc. Wash. 
Acad. Sci., Vol. 7, 1905, pp. 283-288. 

2 Milch, L., Die heutigen ansichten iiber Wesen und Entstehung der kristallinen 
Schiefer: Geol. Rundschau, vol. 1, 1910. 

3 Grubenmann, U., Die kristallinen Schiefer, part 1, 1904, part 2, 1907. 



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78 STRUCTURAL GEOLOGY 

The orientation of the dimensional axes of the particles therefore 
carries with it an orientation of the mineral cleavages. Mica 
crystals, for instance, lying dimensionally parallel in a schist, have 
their mineral cleavages in the plane of the two greater dimensional 
axes, that is, in the plane of rock cleavage. Hornblende crystals 
lie with their long dimensional axes parallel; the mean or least 
dimensional axes of hornblende crystals, being so nearly of the 
same length, may not be parallel. The two cleavages of horn- 
blende are parallel to the major dimensional axes, but are inclined 
to the minor dimensional axes. Thus the hornblende cleavages 
in the schistose rocks are parallel to an axis, but not to a plane. 
The feldspar habit does not give such great dimensional differences. 
Most of the feldspars in schist show only a slight tendency to as- 
sume elongated or tabular shapes due to crystal habit. Their 
dimensional arrangement is more or less independent of cry st al- 
lographs arrangement and therefore there is only a slight tendency 
toward parallelism of the feldspar cleavages. 

The dimensional elongation of mica and hornblende parallel to 
their cleavage faces in schists has been cited as indicating some sort 
of genetic relationship between mineral elongation and mineral 
cleavage. 1 

A schistose rock cleaves either between the mineral particles, 
following the plane of their greatest and mean dimensional axes, 
or within the mineral particles along their cleavage planes. The 
first is known as inter-mineral cleavage, and is a capacity to part 
determined by the dimensional arrangement of mineral particles; 
the second may be called inter-molecular cleavage, and is related 
to the ultimate molecular structure of the crystal. Ordinarily 
when a rock is cleaved the two surfaces show the glistening faces 
of hornblende or mica or of other minerals of this type, indicating 
that the break has followed the mineral cleavages. The parting 
here has obviously been easier than between the mineral particles. 
In places where mica and hornblende are not abundant, the cleaved 
surfaces of the rock show quartz and feldspar, indicating that the 
breaking has been principally of the inter-mineral type. 

Whatever the relative importance of inter-mineral and inter- 
molecular cleavage, it should be remembered that all mineral 

1 Trueman, J. D., The value of certain criteria for the determination of the origin 
of foliated crystalline rocks: Jour. Geol., Vol. 20, 1912, pp. 228-258, 300-315. 



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"Tie " ;.T ^.j 
j IinifJL==. _L= 



iLiinrz: 



!".♦**£. ""*» T . . " - ~ ?~ ~ 

"Ae dr-i.i -i_ - 

TH;CLr Lli Z. ' - — 

of mmt^z* *~ ?- 

of ~m*- 2:»m: — '- 

in* liivr^L -*•'-* 

in^r-iiii»«T vr: .* 

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80 



STRUCTURAL GEOLOGY 



and mica are the common minerals producing the best rock cleav- 
age, it must he concluded that recrystallization is the important 
process in the development of parallelism of the mineral constit- 
uents. 

Corroborative evidence of the importance of recrystallization is 
the general lack of fractures or other strain effects in the minerals 
of a cleavable rock, such as would be expected if the parallelism had 




Ficj. 38. Photomicrograph of micaceous and quartzose schist showing recrystal- 
lizcd quartz. From Hoosac, Mass. The view illustrates in detail the relation 
of recrystallized quartz grains to recrystallized mica flakes. The mica flakes for 
the most part separate different quartz individuals, but they may he seen to 
bound two or more individuals and to project well into them. It is not prob- 
able that such a relation could be brought about by granulation, slicing, or 
gliding, and it seems best explained by recrystallization. 

been brought about entirely or largely by mechanical processes. 
It may be inferred, then, that some constructive process, which 
may be called generally recrystallization, has been at work. 

Most of the mineral particles in the cleavable rocks are in- 
dividually larger than the particles in the same rocks before 
flowage had occurred. For instance, the gradation of a shale to a 
phyllite means an increase in the size of the grains. Recrystalliza- 
tion is the constructive process which has accomplished this result. 



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FLOW CLEAVAGE 



81 



The cleavable rock is likely to show a great uniformity in size and 
shape of the grains of the same mineral as compared with the 
non-schistose rock, and again recrystallization explains the phe- 
nomenon. 




Fig. 39. Photomicrograph of micaceous schist from Hoosac tunnel. The micas, 
which are entirely new developments by recrystallization, lie in flat plates 
with their greater diameters roughly parallel. Each individual exhibits several 
twinning lamella?. It will be noted that, while there is apparently a bending 
and irregularity in the mica plates, the individuals are for the most part not 
deformed, and the impression of irregularity is caused by the individuals 
feathering out against one another at low angles. This sort of arrangement is 
frequently seen about rigid particles which have acted as units during deforma- 
tion, indicating that the arrangement is due to differing stress conditions at 
different places. 

Much detailed microscopical evidence might be cited, such as 
dove-tailing of quartz individuals in quartz bands, the feathering 
out of mica plates against an adjacent mineral surface, the lack of 
bending and breaking of hornblende needles by mutual interference, 
the segregation of minerals into bands, to show that the parallel- 
ism could not have been produced by mechanical adjustment 



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82 



STRUCTURAL GEOLOGY 



1 



alone, but must have been aided by the chemical and mineralogical 
changes involved in recrystallization. (See Figs. 38, 39 and 43.) 

Granulation and rotation of original particles: — But recrystalliza- 
tion is not the only process instrumental in the production of rock 
cleavage. The quartz and feldspar in the cleavable rock may be 




Fig. 40. Photomicrograph of schistose quartz-porphyry showing sliced feldspar 
phenocryst in planes inclined to the prevailing cleavage. After Futterer. 
(Fig. 2, PI. Ill of Ganggranite von Grosssachsen und die Quartzporphyre von 
Thai in Thuringer Wald: Mitt. Grossh. Badischen geol. Landesanstalt, Vol. 2 t 
Heidelberg, 1890.) 



largely original quartz and feldspar; some of the mica and horn- 
blende also may be original. Parallelism may be partly due to 
rotation from original random positions. This process may be 
aided by granulation and slicing of the original mineral particles. 
Broken, unequidimensional mineral fragments are often strewn out 
in such a manner that their longer dimensions lie approximately 
parallel. Evidence of rotation is seen principally in the quartz 



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FLOW CLEAVAGE 83 

and feldspar, which have not much effect in producing rock cleav- 
age. It is concluded, then, that the rotation of original particles, 
diversely oriented, to a parallel position is a minor factor quite 
subordinate to the dominant process of recrystallization. (See 
Figs. 40, 41, 42 and 43.) 

In the incipient stages of rock flowage the larger and more 
brittle particles are granulated and elongated. At the same time 
recrystallization, beginning on the finer particles, builds up new 
minerals. In the intermediate and advanced stages it gradually 
dominates over granulation and ultimately obliterates any evidence 
of it. It may be inferred that granulation aids recrystallization in 




Fig. 41. Sliced feldspars in micaceous and chloritic schist from southern 
Appalachians. 

that it grinds the particles into small pieces and affords greater 
surface upon which the chemical process may act. 

In experimental deformation the conditions are not favorable 
for recrystallization, and granulation is the important process. 

Slipping or twinning along the cleavage planes of minerals, called 
"gliding" — such as may be observed in calcite and ice crystals — 
has been cited as a possible cause of the elongation and parallel 
arrangement of mineral particles. This has been observed only in 
minerals of the calcite type, which are not important in cleavable 
rocks; and even in the calcite of schistose rocks gliding has been 
found to be subordinate to processes of recrystallization and 
granulation. In experimental deformation of marble it seems to 
play a greater part, because conditions of recrystallization are not 
present. 

There is no evidence that the flattening of original mineral 
particles to a dimensional parallelism, without regard to crystallo- 
graphic arrangement, has played any important part in the pro- 
duction of rock cleavage; indeed, some of the facts already cited 



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£4 STRUCTURAL GEOLOGY 

constitute decisive evidence to the contrary. Such is the evidence 
that hornblende and mica, essential minerals of schistose rocks, 
are in many cases, and perhaps in most cases, entirely new develop- 
ments in the rock. Of the same nature is the evidence derived from 
the uniformity of dimensional characteristics of the particles of a 
given mineral species and the control of dimensions by crystal 
habit. The most cleavable rock is not made up of flatter particles 
of hornblende, mica, quartz, or feldspar than the less cleavable 
rock. But it certainly contains more particles of hornblende and 
mica than of quartz and feldspar; consequently it has more parti- 
cles which are flat or elongate, which give it a better and smoother 
cleavage. 

If this is true, the development of rock cleavage would seem to 
require change in chemical composition necessary to increase the 
proportion of the cleavage-making minerals, such as hornblende 
or mica. Chemical evidence seems to the writer to point this way, 
though it is not yet sufficient for proof. 

CLEAVAGE IN ITS RELATIONS TO DIFFERENTIAL PRESSURES 

It has been shown that rock cleavage is determined by the 
parallelism of mineral constituents and that this parallelism is 
developed by rock flowage, which implies differential pressures. 
It now remains to discuss the attitude of cleavage with reference 
to specific pressure conditions. 

What experimental evidence there is indicates that in a non- 
rotational strain (see page 16) mineral particles tend to arrange 
themselves with their longest dimensions normal to the direction 
of the pressure. There is practically no experimental evidence 
bearing on the arrangement of particles under rotational strain or 
shearing, so common in nature. 

Wright 1 melted about 50 grams each of wollastonite, diopside, 
and anorthite, and plunged the melt into water, thereby forming a 
glass. Cubes were then cut from these glasses, heated to a viscous 
state at which crystallization first begins, and subjected to vertical 
pressure. Microscopic examination showed that the three minerals 
named had crystallized with their longer dimensional axes normal 
to the pressure. 

1 Wright, F. E., Schistosity by crystallization. A qualitative proof: Amer. 
Jour. Sci., 4th ser., Vol. 22, 1906, p. 226. 



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FLOW CLEAVAGE 85 

Becker and Day 1 have shown that although crystals are able to 
grow in a given direction in spite of contracting forces, their 
growth in the plane normal to the pressure is vastly greater, 
whether this be the normal direction of elongation due to habit or 
not. Ordinarily in schists the elongation of the crystal is that of 
its normal habit, indicating perhaps that the crystals favorably 
oriented to grow with normal habit have grown at the expense of 
those not favorably oriented. 

Relations of cleavage to strain: — Field observations have to do 
principally with the relation of cleavage to rock strain (see page 14), 
which can be seen, and not with stress t which can not be seen and 
may only be inferred from the strain. After having proved the 
relation of cleavage to strain, the general relations of strain to 
stress may be considered. 

It seems self-evident that the longer dimensions of mineral 
particles in a cleavable rock lie parallel to the elongation of the 
rock mass developed during rock flowage. This relationship has 
been so generally assumed by geologists that at first thought it 
would seem entirely superfluous to present evidence in proof of it. 
But it has been questioned by able geologists. Becker 2 has held 
that the elongation of the rock mass may be inclined to the common 
direction of the major axes of the mineral particles. The student, 
when asked how he knows that cleavage is parallel to rock elonga- 
tion, is often completely at sea. It is simply a matter of observa- 
tion to determine definitely whether the cleavage is parallel to the 
elongation of the mass as a whole. Evidence indicating this 
parallelism is as follows: (1) Distortion of pebbles of a conglomerate. 
Schistose conglomerates show by the distortion of their pebbles 
the plane of elongation, although it may sometimes be difficult 
to distinguish the shapes of undeformed pebbles from those of 
deformed ones. The cleavage of the matrix is approximately 
parallel to the greater diameters of the flattened pebbles, although 
it curves somewhat at the ends of the pebbles. (2) Distortion of 
mineral crystals. The plane of cleavage is marked by mica plates 
or hornblende crystals, while the associated quartz and feldspar 
particles may be fractured at angles with the plane of cleavage. 

1 Becker, G. F. f and Day, A. L., The linear force of growing crystals: Proc. Wash. 
Acad. Sci., Vol. 7, 1905, pp. 283-288. 

2 Becker, G. F., Current theories of slaty cleavage: Amer. Jour. Sci., 4th Ser., 
Vol. 24, 1907, pp. 7-10. 



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86 STRUCTURAL GEOLOGY 

The displacement of the parts, which often accompanies such 
fractures, is observed to extend the fractured parts in the plane 
of rock cleavage. (3) Distortion of volcanic textures. The original 
ellipsoidal parting of basalts frequently shows a flattening, with 
or without fracture; in such cases the ellipsoids and the matrix 
have a flow cleavage parallel to the longer diameters. The elonga- 
tion of amygdules and spherulites in planes parallel to the rock 
cleavage is likewise of common occurrence. (4) Distortion of 
fossils. The elongation of fossils in the plane of cleavage has been 
observed in cleavable rocks. (5) Distortion of beds and attitude of 
folds. Folds often show the direction of shortening of the deformed 
rock mass. (6) Relations to intrusives. Intrusions of great masses 
of igneous rocks, and particularly deep-seated batholiths, exert 
pressure against their walls. Any cleavage developed in the sur- 
rounding rocks is parallel to the periphery of the intrusive masses. 

It is concluded then that the longer dimensions of mineral 
constituents are parallel to the directions or planes of elongation 
of the rock mass. Thus an adequate statement of the relations 
of rock cleavage to the stresses which have produced it must be a 
statement which will cover the various ways in which stress has 
elongated and shortened rock masses. 

Relations of cleavage to stress: — In the simplest possible terms 
stress has been effective in distorting rock masses (1) (see pp. 16- 
21) by non-rotational strain, in which the axes of stress and strain 
remain mutually constant throughout the deformation, and (2) by 
rotational strain in which there is a continuous change in the posi- 
tion of the strain axes as compared with the stress axes during the 
distortion. In the first case the elongation of the rock mass is 
normal to the greatest stress and remains so through the deforma- 
tion; in the second case the elongation of the rock mass is con- 
stantly changing in direction with reference to the principal 
stress, and ultimately the elongation may be considerably inclined 
to the maximum stress. It is held by Hoskins 1 that at any instant 
the tendency for elongation is approximately normal to the greatest 
stress, but that the rotational tendency results in inclining the 
final elongation to the greatest stress. 

Substituting rock cleavage for greatest elongation of the rock 

1 Hoskins, L. M., Flow and Fracture of rocks as related to structure: 16th Ann. 
Kept. U. S. G. S., Pt. I, 1896, pp. 845-874. 



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FLOW CLEAVAGE 87 

mass, the statement of the relations of cleavage to pressure is as 
follows: In a non-rotational strain cleavage is developed normal 
to the greatest stress; in rotational strain, while at any instant 
there may be a tendency for it to be developed normal to the 
greatest stress, there is here a rotational element which brings it 
into position inclined to the greatest stress. All distortional 
strains in rock masses belong to these two classes, rotational and 
non-rotational, and usually to some combination of the two. 
Cleavage, therefore, is developed under some combination of 
rotational and non-rotational strains and may be said to be pro- 
duced both normal and inclined to pressures. 

Specific inferences from field observations as to the pressure 
conditions controlling cleavage are discussed on pp. 119 and 128 
in connection with folds. 

GNEISSIC STRUCTURE 

Gneissic structure means a banding of constituents, of which 
feldspar is important, with or without the parallel dimensional 
arrangement necessary for rock cleavage. A schist always has a 
parallel dimensional arrangement and may or may not contain 
feldspar. A gneiss may or may not have a parallel arrangement, 
but always has a banding and contains feldspar. So far as this 
parallel arrangement is present, gneissic structure has been dis- 
cussed under the heading of rock cleavage. In many cases, how- 
ever, cleavage in gneisses is not good. The essential mineralogical 
difference between gneisses and schists is the possession by the 
gneisses of a relatively small amount of the platy and columnar 
minerals so necessary for a good rock cleavage, and correspond- 
ingly more feldspar and quartz. 

The origin of perhaps the majority of gneisses is not yet known. 
In a few instances the structure has been identified as an original 
magmatic flow structure, the "protoclastic" structure. In other 
cases it is the result of secondary rock flowage, either of igneous or 
sedimentary rocks, the "crystalloblastic" structure. Some criteria 
which have been useful in discriminating the igneous and sedi- 
mentary gneisses resulting from rock flowage are the broader 
field relations, the chemical composition, the possession of igneous 
or crystalloblastic textures, and the content and form of the heavy 



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88 



STRUCTURAL GEOLOGY 




Fig. 42. Photomicrographs showing the progressive granulation of the Morin 
anorthosite under the influence of pressure. After Adams. 



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GNEISSIC STRUCTURE 



89 



residues such as zircon. 1 These criteria may often be decisive when 
applied collectively, but seldom when used separately. They are 
discussed on a subsequent page (97). 

Gneisses have been known to develop by rock flowage from 
rocks which under other conditions have yielded schists. What, 




Fig. 43. Photomicrograph of leaf gneiss from the Laurt ntian area north of Mon- 
treal. Slide furnished by Frank D. Adams. Doctor Adams has described the 
leaf gneiss as resulting from granulation of a hornblende granite, all stages of 
the process having been noted. (See Part J of Vol. VIII of the Geological 
Survey of Canada, 1895.) The striated feldspars have irregular angular shapes 
such as characteristically result from granulation. The two bands of quartz 
crossing the slide evidently owe their form and arrangement finally to recrystal- 
lization, although granulation may have been an important initial process. 
It will be noted that the quartz individuals have dimensional but not crystal- 
lographic parallelism. 

then, are the conditions which determine whether the gneissic 
or the schistose structure will result from the rock in question? 
Study of all analyses available of schistose and gneissic rocks 

1 Trueman, J. D., The value of certain criteria for the determination of the origin 
of foliated crystalline rocks: Jour. Geol., Vol. 20, 1912, pp. 228-258, 300-315. 



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90 STRUCTURAL GEOLOGY 

indicates a higher percentage of moisture for the schists than 
for the gneisses. The moisture is concentrated largely in the 
tabular and columnar minerals which so largely determine rock 
cleavage. Many schists are found along shearing planes in non- 
schistose types. Water has been allowed access here by means 
of the fractures. Other schists represent anamorphosed sedi- 
ments which originally contained a good deal of water. The 
principal change in the development of secondary gneissic struc- 
ture is one of granulation and recrystallization of substances 
present, not the development of new tabular or columnar minerals 
requiring water. A good illustration is the case of the sheared 
anorthosites, or gneisses, described by Adams * as developing from 
fresh anorthosites entirely by granulation accompanied by a 
minimum of recrystallization and consequent development of 
hornblende and mica. (See Fig. 42.) Adams also has experimen- 
tally deformed diabase, under dry conditions unfavorable for 
recrystallization. The deformation was principally by granula- 
tion. The result was a gneiss. 

If water is essential to the development of the best cleavage- 
giving minerals, it may be argued that its absence may be respon- 
sible for the lack of development of a good cleavage during rock 
flowage. Although it is not proved, it seems entirely plausible that 
many of the gneisses, especially if they developed from granite, 
have been formed under deep-seated conditions unfavorable either 
to the original presence of water or to its introduction during 
deformation. There may be other and more decisive factors, as 
yet unknown. 

IDIOMORPHIC OR PORPHYRITIC TEXTURES DEVEL- 
OPED BY ROCK FLOWAGE 

Garnet, staurolite, tourmaline, andalusite, chloritoid, and 
other heavy anhydrous minerals of this kind are uniformly idio- 
morphic or porphyritic in cleavable rocks. They develop by 
recrystallization after rock flowage has ceased, but probably 
while the rock is still under high pressure and temperature, as 
is evidenced by their high specific gravity and characteristic 

1 Adams, F. D., Report on the geology of a portion of the Laurentian area lying 
to the north of the island of Montreal; Ann. Rept. Geol. Survey of Canada, Vol. 8, 
pt. J, 1896, p. 85 et seq. 



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PORPHYRITIC TEXTURE 



91 



occurrence in the proximity of intrusive igneous rocks. Their 
late development by recrystallization is shown by the following 
considerations: (1) They appear in rocks clearly derived by rock 
flowage from others originally lacking such minerals. (2) They 
frequently lie at large angles to the prevailing cleavage in the 
rock. (3) They do not show the degree of mechanical deforma- 
tion that they would necessarily have possessed had they devel- 
oped in their present positions before flowage had ceased. Many 




Fig. 44. Photomicrograph of chloritoid crystal in micaceous and quartzose schist 
from Black Hills. The chloritoid crystal here shown has developed later than 
the rock flowage producing the prevailing cleavage of the rock. The chloritoid 
has grown at the expense of the other constituents of the rock, using all the 
material necessary for its growth and leaving the excess of material in the form 
of inclusions, which retain their dimensional parallelism with the prevailing 
rock cleavage. 

of the crystals are long and acicular, and would surely have been 
broken if any considerable movement had occurred subsequent 
to their development. (4) They include, within their boundaries, 
minerals in part similar to those in the remainder of the rock, 
and which have an arrangement of their greater diameters in the 



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=_ I 1 • Y 

_r - -* czrr >*£nee at least they 

.2- " T:^ zl:-^ and the other 

t „ •: r 1 -^rrainly developed 

-- • — ? x :>^: -rmation, are fre- 

_- *»«rT»itrT- x a mineral of this 

. r ^ ~_>7 :c:efi do about the 

^ -„rr r»-i i^i 5 .•wed after the 

-^__~ t- ^r -j ii. i tending of the 

;r- -~ * Tiir us^al large size 

„--. t~_i -j^-zr is-^xiated mineral 

n-i~ -~:-^ jtcz :o rock flowage, 

i ir.-c v :rv^i ir-wn the crystals. 

r £~ »iz .c :ry^jils is believed to 

niLL-j ci :c :he cleavage, it is 

— if-iz £t_T-2j?r has resulted in 

^x *_ .*? o-r !*;*rtst:tuents. The 

.-r lit "^nt^i: ire so conspicuous 

— - cl-z^tt T^crhyritic minerals 

-. i-~tr T^e n>:»vement ceased. 

^- - ci^-jiiiS of this peculiar 

:r~L^- —^7 sre probably high 

:. ^^-ti- - z*: lirenential stress, 

— it-- i^ji *^zi^rature may be 

,r^:* is v irv-rlop hydrostatic 

ihf-r-TinL pressures necessary 

-:~.trr:s *zj£ ilis might afford a 

--- -rcnsr: :c ~bese non-oriented 



^ rr i.rrzsr:;.y of cleavage 

— r ^^i;^ ;c i rxk which undergoes 
:c -t :j^- It :c~fn occurs between 
* r *--.. T T -^;ir: 3:<ir: : he marble itself 
^-^s 3>; rle-LTi^r. Cleavage may be 
n uar-'i*f :j Ttr^ssur^ alone, when the 
- r r^;r^r>„ c ration. 1 Microscopic 

~- ~. *-i ~ i *z*'Trm***ir> i : i^T^siation into the flow 

.J. ^ a. V u Is?. Ir* I. re- 363-401. See also 

'.. ~i*r i».*» .'£ lus^-rLt Atmt- Jour. Sci., Vol. 29, 



'"""Dt&iT 



FLOWA- £ T :~ -: T 1 

examination ini: -.*•■?- *i.«- * :_- : .- — i 
illation, slicing, an : £_• . :r r - :e - . - - - 
cleavage is ot»^€Tv«ev: _l n. :_- ^ > '-ii*-: n: 
It may be suppo-^i :ii* z^ul - r..:.~ — :— 
in the early sT^ef :** "I-^jt t. v _u- 
easily that the para^r-l ~r-/ — _-* • . _— . * r 

is soon destroy e»i. Tir >— •* :-- •-.. ' 

the habit necessary : < ± r • •: _-::»-•:- •:... 
So far as the Iin>=— - • o-^ :.: * n:.:^— 
silicates are likely v_. _-- - - -_••: :- ^ • 
which by their arrar.irecjT'L: r— * n * ~ :.- 

OBLITERATION OF TZ_\7~ J ^ 2 . 

Recrystallizatioii- ::- :• ci-..:* :^ • — 
toward an increu-*- :i. *:.- --_>- r xr. .:. " - - 
into bands, a un::«>m.:*y ir_ --_>- i>:i. - ■.. -*• * 
and the growth of i~tt rr -^-_- -.< : ~* :.. 
previously exist en* ir. *:- ; • t_ }—- » • -- 
destroyed. Bed«i:r-£ i- .♦.•-__.; ij •** •• •:: »•-- 
alternation of b*>> of orvs:^. - = " — \" 
and texture determine v. - ---in- *-:---:- - :»* 
secondary' mineral prj^ - :; .-^ : : .r_y. ;i * > — ► 
Thus a faint band:i*£ of i-"£ r r ' i: :»— . 

minerals may mark tLe or-ju: •#- J _.:^ :: . 

Figs. 45, 46 and 47, . 




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U2 STRUCTURAL GEOLOGY 

plane of rock cleavage, showing that to some degree at least they 
were formed during rock flowage. (5) The mica and the other 
constituents of cleavable rocks, which are certainly developed 
by reerystallization during the process of deformation, are fre- 
quently seen to end abruptly at the periphery of a mineral of this 
group and not to curve around it as they often do about the 
resistant minerals in schists. If the rock had flowed after the 
formation of the porphyritic crystals, crowding and bending of the 
micas must inevitably have occurred. (6) The usual large size 
of minerals of this group, as compared with their associated mineral 
particles, suggests their development subsequent to rock flowage, 
when granulation is no longer tending to break down the crystals. 

While the development of this group of crystals is believed to 
have been mainly later than the formation of the cleavage, it is 
true also that in some cases subsequent flowage has resulted in 
their being fractured and crowding the other constituents. The 
very fact that the effects of further movement are so conspicuous 
confirms the conclusion that the secondary porphyritic minerals 
not showing these effects developed after the movement ceased. 

We may only speculate as to the conditions of this peculiar 
development of non-arranged minerals. They are probably high 
temperature and pressure, but apparently no differential stress, 
requiring movement. If the pressure and temperature may be 
considered as having become so great as to develop hydrostatic 
conditions, there would be no differential pressures necessary 
for a parallel arrangement of constituents and this might afford a 
plausible explanation of the development of these non-oriented 
porphyritic minerals. 

ROCK FLOWAGE WITHOUT RETENTION OF CLEAVAGE 

Marble is the commonest example of a rock which undergoes 
flowage without retaining cleavage. It often occurs between 
schistose beds which have flowed, without doubt the marble itself 
has flowed, and yet it possesses no cleavage. Cleavage may be 
produced experimentally in marble by pressure alone, when the 
conditions are not favorable for reerystallization. 1 Microscopic 

1 Adams, F. D., and Nioolson, J. T., An experimental investigation into the flow 
of marble: Phil. Trans. Roy. Soe. of London, Vol. 195, 1901, pp. 363-401. See also 
Adams, F. D., and Coker, E. G., The flow of marble: Amer. Jour. Sci., Vol. 29, 
1910, pp. 465-487. 



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FLOWAGE WITHOUT CLEAVAGE 93 

examination indicates that this has been accomplished by gran- 
ulation, slicing, and gliding of the calcite crystals. Rarely such a 
cleavage is observed in marble deformed under natural conditions. 
It may be supposed that many marbles have shown this structure 
in the early stages of their flowage, but calcite recrystallizes so 
easily that the parallel structure caused by mechanical deformation 
is soon destroyed. The recrystallized calcite crystals do not have 
the habit necessary for a good dimensional arrangement in schists. 
So far as the limestones have impurities in them, secondary 
silicates are likely to develop, such as actinolite and tremolite, 
which by their arrangement may give the rock a cleavage. 

OBLITERATION OF TEXTURES BY ROCK FLOWAGE 

Recrystallization, the dominant process in rock flowage, tends 
toward an increase in the size of grain, the segregation of minerals 
into bands, a uniformity in size and shape of the mineral particles, 
and the growth of new minerals such as mica or hornblende not 
previously existent in the rock. Previous textures are commonly 
destroyed. Bedding is locally not completely obliterated, because 
alternation of beds of originally different mineralogic character 
and texture determines to some extent the kinds and size of the 
secondary mineral particles formed in these beds by rock flowage. 
Thus a faint banding of dark or light minerals or of fine or coarse 
minerals may mark the original bedding in a schistose rock. (See 
Figs. 45, 46 and 47). 



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94 



STRUCTURAL GEOLOGY 




Fig. 45. Photomicrograph of micaceous and quartzose schist with cleavage de- 
veloped across original bedding, from Little Falls, Minn. A graywacke-slate, 
in which the banding has been marked by difference in texture as well as in 
composition, has been subjected to deformation, with the result that a cleavage 
has been superposed at right angles to the original bedding. Originally the 
longer diameters of the particles of the bedded rock were parallel to the bed- 
ding. Accompanying the development of flow cleavage most of the con- 
stituents of the rock have been recrystallized. The quartz particles shown in 
the light band have been drawn out with their longer diameters nearly at right 
angles to the former plane of their longer diameters, and abundant new mica 
has developed with its greater diameters and mineral cleavage normal to the 
plane of bedding. 



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FLOW CLEAVAGE AND BEDDING 



95 




Fig. 46. Cleavage crossing bedding of slates, St. Louis river, Minnesota. The 
broad plane surface dipping to the right is a bedding plane. The structure 
dipping more steeply to the right is cleavage. 



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96 



STRUCTURAL GEOLOGY 




Fig. 47. Slaty structure and its relation to bedding planes. Two miles south of 
Walland, Tenn. After Keith. 



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ROCK FLOW AGE 97 

IDENTIFICATION OF SCHISTS AND GNEISSES 

Not only does rock flowage tend to obliterate primary textures 
but it modifies the chemical and mineralogical composition. 

In proportion, then, as rocks have undergone rock flowage, there 
may be difficulty in ascertaining their origin. The identification of 
the origin of schists and gneisses is more largely a metamorphic 
than a structural problem, but it is difficult to separate the two 
phases of the problem. Both are covered in the following sum- 
mary of criteria. 

FIELD RELATION AS A MEANS OF IDENTIFYING SCHISTS AND 

GNEISSES 

Field and microscopic observation of gradations from unde- 
formed rocks into schists or slates gives certain empirical methods 
for recognition of origin of some schists and gneisses. For instance, 
a shale alters to a slate and this in turn to a phyllite. While it is 
difficult from the study of the phyllite alone to determine its 
origin, it so often has been observed as the end-product of this 
series of changes that there is little danger of mistake if it is re- 
ferred back to a shale or a mud. A sandstone or quartzite may be 
traced into a mica-quartz-schist, seldom into a hornblende schist. 
A similar schist may be derived from the secondary deformation 
of certain acid igneous rocks. A quartz-mica-schist therefore is 
regarded as the natural development of an acid rock, but whether 
sedimentary or igneous may be doubtful, when field relations do not 
decide. A basalt is observed to grade into a chloritic and micaceous 
schist. The same result may be observed where certain shales 
are altered. Basic igneous rock (especially in the vicinity of 
intrusives) by rock flowage may pass into coarsely crystalline 
hornblende schists or gneisses. Amphibolites are known to be 
formed also by alteration of limestone. Some banded gneisses, by 
their association with, and gradation to, granites, and by their 
mineralogical composition, seem to be surely the result of rock 
flowage of a granite, though cases of proved gradation are rare. 
It has been observed, however, that certain sediments, such as an 
impure quartz sand, have gone over to gneisses with general as- 
pects similar to those presumably developed from a granite. The 
passage of a dolomite into a talc schist is not uncommon. 



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98 STRUCTURAL GEOLOGY 

Schists or gneisses may l>e interbedded with sediments and 
be themselves in lieds strongly suggestive of sedimentary origin. 
They may l>e in an igneous complex and have irregularity of form 
or distribution or relations to adjacent rocks more characteristic 
of an igneous mass than of sedimentary beds. Some gneisses of 
the Laurentian are clearly original igneous rocks intrusive into 
adjacent rocks. Where the schist or gneiss shows marked differ- 
ences in composition in different beds or bands, and this composi- 
tion is persistent throughout these bands for long distances, it is 
suggestive of sedimentary origin, especially if some of the beds have 
mineral or chemical composition of sediments. The Baltimore 
and Carolina gneisses of the Piedmont Plateau, 1 and the Idaho 
Springs formation of the Georgetown area of Colorado 2 are of this 
type. Yet analogous structure has been produced, perhaps on a 
smaller scale, by injections of igneous masses along parallel planes. 

On the whole, with our present knowledge, field observations 
are likely to yield more satisfactory conclusions as to origin than 
other criteria below discussed. 

MINERAL COMPOSITION AS A MEANS OF IDENTIFYING SCHISTS 

AND GNEISSES 

A great preponderance of quartz is perhaps more often charac- 
teristic of a sedimentary than an igneous rock. Where a gneiss 
or schist is dominantly quartz, one looks for other evidences of 
sedimentary origin. But the existence of highly quartzose rocks of 
the pegmatite and alaskite types makes quartz content alone a 
doubtful criterion. Preponderance of calcite is more satisfactory 
evidence of sedimentary origin. 

The abundant development of aluminum silicate minerals 
such as staurolite and sillimanite 3 has been more commonly ob- 

1 See: Keith, Arthur, Washington folio (No. 70), Geol. Atlas U. S. t U. S. Geol. 
Survey, 1900. 

Mathews, E. B., Correlation of Maryland and Pennsylvania Piedmont forma- 
tions: Bull. Geol. Soc. Am., Vol. 16, 1905, pp. 329-346. 

Bascom, F., Piedmont district of Pennsylvania: Bull. Geol. Soc. Am., Vol. 16, 
1905, pp. 289-328. 

2 Spurr, J. E., and Garrey, G. H., Economic geology of the Georgetown quad- 
rangle, Colorado: Prof. Paper U. S. Geol. Survey No. 63, 1908, p. 44. 

3 Emmons, W. H., and Laney, F. B., Preliminary report on the mineral deposits 
of Ducktown, Tenn.: Bull. 470, U. S. Geol. Survey, 1911, p. 158. 

Spurr, J. E., and Garrey, G. H., Economic geology of the Georgetown quad- 
rangle, Colorado: Prof. Paper U. S. Geol. Survey No. 63, 1908, p. 44. 



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IDENTIFICATION OF SCHISTS AND GNEISSES 99 

served in metamorphosed sediments than in igneous rocks. Any 
of these minerals, however, may be found also in igneous rocks. 

Where gneiss is strongly feldspathic, it is not likely to be re- 
garded as of sedimentary origin. Yet so far as the sediment is 
undecomposed, it may be largely feldspathic, and also the anamor- 
phism of a nonfeldspathic sediment might make it feldspathic, 
though it is a questipn whether to a degree common to many 
gneisses. 

The presence of graphite disseminated evenly through a band 
or zone becomes presumptive evidence of sedimentary origin, 
especially where, as in the Adirondack graphites, there are other 
evidences present. 1 Some graphite may be igneous in origin, but 
when evenly distributed in amount up to about 6% in a generally 
slaty or quartzose zone, the hypothesis of igneous origin becomes 
untenable. 

Mica or chlorite or hornblende affords no satisfactory criterion 
of identification of origin, for these minerals develop both from 
sedimentary and from igneous rocks. But so far as present evi- 
dence goes, they seem to develop more readily from sediments 
than from igneous rocks, perhaps because water is necessary. 
This criterion must be most carefully used, in view of the fact 
that sedimentary composition may be approached by the weather- 
ing of igneous rocks prior to anamorphism. The basalts of the 
Menominee district described by George H. Williams alter by 
katamorphism into chloritic rocks and under pressure alter to 
chlorite-schists. The mineral change from the fresh rock is the 
same in both cases. It may be that the chlorite-schist was pre- 
ceded by katamorphism of the basalt. 

The separation of minute accessory constituents by washing is a 
means for identifying origin which has not yet been sufficiently 
used. In deeply weathered rocks like those of central Brazil this 
method has been used effectively by Dr. Derby and associates in 
determining whether the weathered material is igneous or sedimen- 
tary. 2 Minerals of igneous rocks like monazite, zircon, sphene, 
garnet, and so on, are remarkably resistant to weathering, and will 
remain in well defined crystals when all the other constituents have 

1 Bastin, E. S., Origin of certain Adirondack graphite deposits: Econ. Geol., 
Vol. 5, 1910, pp. 134-157. 

2 Derby, O. A., On the separation and study of the heavy accessories of rocks: 
Proc. Rochester Acad. Sci., Vol. 1, 1891, pp. 198-206. 



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100 STRUCTURAL GEOLOGY 

altered. When these are found unmodified in the weathered rocks, 
it is assumed that the reck is of igneous origin. The argillaceous 
sediments lack these substances. Quartzites may possess them, 
but they are there likely to show distinct wearing by attrition. 
In the schistose equivalent of the quartzite the rounded grains 
persist, particularly in zircon. Where, therefore, in an argillaceous 
schist these heavy accessory minerals are lacking or in a quartz 
schist are rounded, a sedimentary origin is probable. 1 

CHEMICAL COMPOSITION AS A MEANS OF IDENTIFYING IGNE- 
OUS OR SEDIMENTARY ORIGIN OF GNEISSES AND SCHISTS 

If the composition of a schist or gneiss is substantially that of an 
igneous rock, their igneous origin is usually regarded as probable 
at first thought, yet the basis for this supposition is an unsatis- 
factory one, for so far as sediments are produced from igneous 
rocks by disintegration rather than decomposition, the primary 
composition of the sediments approaches that of the igneous 
rocks. Also facts have been found to show that the general tend- 
ency of anamorphism of sediments is toward the reproduction of 
the composition of igneous rocks, both by dynamic and contact 
metamorphism. The tendency is not known fully to accomplish 
this result, but certainly it approaches it closely enough to give a 
composition which is not so different from that of the igneous rock 
that it may be certainly classed as sedimentary. So far as quanti- 
tative evidence yet goes, igneous composition of a schist may 
indicate igneous origin, it may indicate that the schist came from a 
sediment of igneous composition, or it may represent an extreme 
of anamorphism of sediments which has tended to reproduce igne- 
ous composition in them. 

If the composition of the schist or gneiss is that of a sedimentary 
rock, it has been somewhat generally assumed that this proves the 
sedimentary origin of the schist or gneiss. 2 Distinctive features 
of sedimentary origin, as summarized by Bastin, 3 are dominance of 

1 Trueman, J. D., The value of certain criteria for the determination of the origin 
of foliated crystalline rocks: Jour. Geol., Vol. 20, 1912, pp. 244-258. 

2 Adams, F. D., Geology of a portion of the Laurentian area lying to the north 
of the island of Montreal: Ann. Rept., Geol. Survey of Canada, Vol. 8, part J, 1896, 
p. 57 et seq. 

Adams, F. D., and Barlow, A. E., Geology of the Hailburton and Bancroft areas, 
Ontario: Geol. Survey Can., Mem. No. 6, 1910. 

3 Bastin, Edson S., Chemical composition as a criterion in identifying meta- 
morphosed sediments: Jour. Geol., Vol. 17, 1909, p. 472. 



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IDENTIFICATION OF SCHISTS AND GNEISSES 101 

magnesia over lime, of potassa over soda, excess of alumina, and 
high silica. To these must be added all other known chemical 
peculiarities of sediments. These criteria should be used with 
knowledge and consideration of the general chemical processes 
involved in the development of sediments. However, so far as an 
igneous rock is katamorphosed before or after it becomes schistose, 
its composition approaches that of a sediment, in which case the 
composition might be that of a sediment and yet the rock may 
never have been a sediment. Bastin recognizes this possibility, 
but considers it of minor significance. Some schists and gneisses 
develop from igneous rocks and retain original igneous composi- 
tion. It is known that others do not. It has not yet been proved 
which is the common case, but quantitative evidence is less satis- 
factory for the former than for the latter. Plutonic rocks may be 
less katamorphosed than volcanics prior to anamorphism, but 
direct evidence of this is not available. In addition to surface 
weathering it is necessary to include all hydration and solution 
which may take place in the zone of fracture, and also hydrother- 
mal alteration which has essentially the same chemical effect as 
weathering as far as lime-magnesia and soda-potassa ratios are 
concerned. The conclusion that a sedimentary composition of a 
gneiss or schist means sedimentary origin is based simply on the 
fact that some igneous rocks become schistose or gneissic without 
change in composition and ignores the equally well established fact 
that others have approached the sedimentary rocks in composition 
prior to or during or after the alteration to schist or gneiss. Appli- 
cation of the chemical criteria for sedimentary origin outlined by 
Bastin to the green schists of the Menominee district of Michigan, 
shown by Williams 1 and others to be largely schistose basalts, 
illustrates the uncertainty of these criteria in determining origin. 
Under these criteria, part of Williams' analyses are those of sedi- 
ments, part are those of igneous rocks, and part have intermediate 
characters. 

Chemical composition, therefore, in the present state of knowl- 
edge, must be regarded as an extremely uncertain basis for deter- 
mining igneous or sedimentary origin. If the composition is that 
of an igneous rock, it is plausible to assume that the probability 

1 Williams, G. H M The greenstone schist areas of the Menominee and Marquette 
regions of Michigan: Bull. 62, U. S. Geol. Survey, 1890. 



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102 STRUCTURAL GEOLOGY 

slightly favors igneous origin, but the same composition may be 
possessed by a sedimentary rock, either because of its primary 
character or because of composition which has been induced in it by 
anamorphism. If the composition of the schist or gneiss is that of 
a sedimentary rock, the balance of probability would perhaps 
slightly favor its sedimentary origin, but igneous rocks are known 
also to take on this composition, either prior to or during their 
anamorphism. When vastly more chemical analyses of well 
selected sets of rocks become available to show specifically the 
range of chemical changes in anamorphism of both igneous and 
sedimentary rocks, it may be possible to use chemical criteria 
which will aid in determining the origin of the schists and gneisses. 

CONCLUSION AS TO METHODS OF IDENTIFYING GNEISSES AND 

SCHISTS 

The writer knows of no case where all the evidences above cited 
have been used in the determination of sedimentary origin of a 
gneiss. As one surveys the methods used in the conclusions 
reached in various investigations of gneisses and schists, it be- 
comes apparent that no one criterion is sufficient to establish 
sedimentary origin. 

Gneisses developed secondarily from igneous rocks by pressure 
and recrystallization have been positively identified in even fewer 
cases than sedimentary gneisses. Many gneisses are known to be 
original igneous rocks with flow structure; a few have been found to 
be the result of mechanical breaking down by granulation, for 
instance, the granulated anorthosites described by Adams. 1 
Many gneisses have been described as the foliated equivalents of 
granites as result of pressure and recrystallization, but often 
without adequate proof of this relation. Lehmann has appar- 
ently shown the development of gneisses from granites in the 
Saxony area. In parts of the Lake Superior country there are 
gneisses which seem to have such relations to granite gneisses as 
would result from secondary pressures and recrystallization, but 
there is not a single proved case there. Many pairs of analyses of 
granites and equivalent gneisses have been published, but these 

1 Adams, F. D., Report on the geology of a portion of the Laurentian area lying 
to the north of the island of Montreal: Ann. Rept. Geol. Survey Can., Vol. 8, part 
J, 1896, p. 85 et seq. 



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IDENTIFICATION OF SCHISTS AND GNEISSES 103 

have usually been made on the assumption that the gneiss was the 
result of secondary alteration of granite and without adequate 
consideration of the possibility that gneissose structure may be 
an original flow structure. 

Many more schists than gneisses have been proved to be the 
result of mashing of igneous rocks, for instance, the chlorite schists 
so commonly developed from the mashing of basalt, illustrated by 
the schists in the Keewatin series of the Lake Superior country; 
the hornblende schists formed in these rocks by contact metamor- 
phism of granites; micaceous schists formed in granites and por- 
phyries along a shear zone. In fact, so commonly do the igneous 
rocks appear when mashed to take on schistose as contrasted with 
gneissic structure as to raise the question whether this is not the 
common result of mashing and whether gneisses are not exceptional 
results, most gneisses to be explained as igneous rocks with original 
flow structures. 

This brings us back to a suggestion made on an earlier page, 
that when igneous rocks break down by mashing, there tend to 
develop the platy and columnar hydrous minerals characteristic 
of schists. These minerals are the same in kind as those derived 
from the anamorphism of a sediment. As compared with the 
igneous rock, the change to a schist amounts to katamorphism, and 
requires the introduction of water and carbon dioxide. To what- 
ever extent gneiss may be formed by the mashing of igneous rocks, 
and, as noted, this extent is extremely problematic, conditions 
different from those forming schists are implied by the fact that the 
gneisses have relatively less amounts of platy and columnar min- 
erals and the change has obviously been under conditions not 
those of hydration and carbonation, or katamorphism in general. 
We have suggested that gneisses may form only in places where 
the agents of hydration and carbonation are lacking, and that 
where these agents were present, the change is more toward the 
schist type. 

The terms " schist" and "gneiss" have been used as representing 
two contrasting types of rocks. It is of course to be recognized 
that there are complete gradations between schist and gneiss; that 
it probably follows therefore that there are many conditions of 
origin of the schists and gneisses from igneous rocks intermediate 
between those described. 



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STRUCTURES COMMON TO BOTH FRACTURE 
AND FLOW 

FOLDS 

ELEMENTS OF FOLDS 

The elements of a simple fold are indicated in the following 
diagram (Fig. 48) taken from Willis. 

The attitude of a rock bed is described in terms of strike and dip. 
Strike is the direction of line of intersection of the bed with the 
horizontal; dip is the angle between the bed and the horizontal, 



o- PITCH 




^flTC* 

Fig. 48. Parts of folds. After Willis. 

measured at right angles to the strike. Folds are usually deter- 
mined by the correlation of strike and dip observations. 

The axial plane of a fold intersects the crest of trough in such a 
manner that the limbs or sides of the fold are more or less symmet- 
rically arranged with reference to it. The intersection of the 
axial plane with the crest or trough of a fold is the axial line, axis, 

104 



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ELEMENTS OF FOLDS 105 

crest line, or trough line. The pitch of the fold is the inclination of 
the axial line to the horizontal. It is merely a special case of dip 
taken along the axis. 

Strike and pitch are never strictly parallel, although if the pitch 
is slight, they may be nearly so. 

A simple fold is a single bend or curve without minor crenula- 
tions. A composite fold is the simple fold with minor crenulations 
superposed on it. A complex fold is one which is cross folded, 
that is, one of which the axial line is folded. As defined by Van 
Hise, composite refers to two dimensions, or the cross section, and 
complex to three dimensions. 1 

As practically all rock folds are complex, it appears that the 
terms "simple" and "composite" merely apply to descriptions 
of cross sections of complex folds. It is not always easy in dis- 
cussing folds to discriminate clearly between a consideration of 
two dimensions and of three dimensions, and hence the use of the 
terms "composite" and "complex" is in practice frequently loose. 
The terms are useful, however, in keeping clearly before us the 
desirability of discrimination between two-dimension and three- 
dimension treatment of folds. 

The axes of minor folds may have almost any angle with refer- 
ence to the axis of the major fold, but there is a marked tendency 
to have a similar angle of pitch and a constant, though small, 
difference in strike. 

Anticline and syncline refer respectively to the arch and trough 
of a simple fold. Anticlinorium and synclinorium refer to com- 
posite arches and troughs. Some of the great simple flexures of the 
earth have been called by Dana geanticlines and geosynclines. 2 

Each of these kinds of folds may be further classed as upright, 
inclined, overturned, or recumbent, depending upon whether its 
axial plane is vertical, inclined, overturned, or recumbent. No 
further definitions of these terms seem necessary. Where the 
limbs of a fold are parallel, it is called isoclinal. When the axial 
planes of the minor folds of an anticlinorium converge downward, 
the fold is called by Van Hise a normal anticlinorium; a fan fold 
is a special case of this (Fig. 49). If they converge upward it is 

1 Van Hise, C. R., Principles of North American pre-Cambrian geology: 16th 
Ann. Rept., U. S. G. S., part 1, 1896, p. 603 et seq. 

2 Dana, James D., Manual of geology, 4th ed., 1895, p. 106. 



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106 



STRUCTURAL GEOLOGY 



called an abnormal anticlinorium; roof structure is a special case of 
this (Fig. 50). A similar division applies to synclinoria. 

Minor folds are commonly developed in weak beds by the 
shearing between two more competent masses of rock. These 




Fig. 49. Generalized fan fold or normal anticlinorium of central massif of the Alps. 

After Heim. 

folds are conveniently designated drag folds. The position of their 
axial planes is controlled by the displacement of the more com- 
petent beds adjacent. The term "drag fold" is desirable as 
emphasizing the differential movement between the controlling 
beds. 




Fig. 50. Generalized section of roof structure or abnormal anticlinorium of the 
central massif of the Alps. After Heim. 

A parallel fold (Fig. 51) is one in which there is no thickening or 
thinning of the beds; the bedding surfaces are mutually parallel. 
The curvature of no two beds is exactly the same. This difference 
in curvature implies the dying out of folds in one direction or 
another from a given bed, and the differential slipping between the 
layers to allow for the dying out and differing curvature. 

In similar folds (Fig. 51) the beds are thickened and thinned, the 
bedding surfaces are not mutually parallel, but the curvature Is 
the same for all beds. This does not require the dying out of folds 
or differential movement between beds 



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ELEMENTS OF FOLDS 



107 




Fig. 51. Figures illustrating (a) ideal parallel and (b) i'U-*i *vf< *' *'.''t* ' '' * 

Van Hise. 



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108 



STRUCTURAL (JEOLOGY 



FOLDS IN THK ZONE OF FRACTURE AND ZONE OF FLOW CON- 
TRASTED 

Rocks are folded by fracture or flow or by any combination 
of these two processes. Folds therefore appear in either the zone 
of fracture or the zone of flow or in the zone of combined fracture 
and flow. Folding by fracture differs in certain essential charac- 
teristics from that by flowage. 




Fig. 52. Folding of brittle and soft layers contrasted in jasper. The broken dark 
layers are chert, the light layers are secondary iron oxide. 

Folds may be formed by means of minute displacements along 
numerous joints and faults. Folds in brittle quartzite beds are 
commonly of this type. 

There is no interior deformation of the fault and joint blocks, 
and there is no thickening or thinning of the beds as a whole. The 
top and bottom of a bed are parallel throughout. The fold is of 
the "parallel" type. The curvature of the beds so folded is not 



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FOLDS OF FRACTURE AND FLOW CONTRASTED 109 

the same through any considerable vertical distance. A much 
folded bed may be replaced above or below, usually below, by a 
much less folded bed or one which is deformed almost none at all; 
in other words, there is a dying out of the fold. Disappearance of 
folds with depth is discussed on pages 124-127. The difference in 
the shortening of the adjacent strata involves slipping between the 
beds. This slipping is really of the nature of faulting, although the 




Fig. 53. Folding of brittle and soft layers contrasted in jasper. Note the tension 
cracks in the brittle layers. 

movements are not ordinarily described as faults, on account of 
taking place parallel to the bedding. 

In the zone of fracture rocks are relatively competent; they 
do not crumple by interior adjustment; the folds therefore tend to 
be simple and open. 

In the zone of flowage rocks are folded by interior adjustment of 
all parts of the mass with development of cleavage. Beds are 
thickened and thinned. No part of the rock mass is competent to 
withstand the load without interior adjustment and crumpling. 
The result is a much more composite or complex folding. The 
bed thereby becomes thickened and strengthened, enabling it to 



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a 

1 



3 



110 



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FOLDS OF FRACTURE AND FLOW CONTRASTED 111 

support the load. The folding of rocks in schistose areas, that is, 
areas which indicate that they have been in the zone of flowage, is 
intricate and close, and contrasts strongly with the more open and 
simple folding of rocks in the zone of fracture. For instance, the 
folding in the Piedmont area of Virginia, the rocks of which were 
deformed in the zone of flowage, is much more minute and com- 
plex than that of the Knox dolomite in the Appalachians to the 
west, which occurred partly in the zone of rock fracture. In the 
folds of the zone of flowage the readjustment takes place not 
only between the beds but in every part of the bed. The curva- 
ture in each bed tends to remain the same as in the strata above 
and below. This is called the " similar " type of folding. (See 
Figs. 51b and 54.) The distortion in the layers in ideal similar 
folds is greater in proportion as the bends are gentle on the anti- 
clines and synclines. Hence, to avoid this distortion, there is a 
tendency for very sharp turns at these places. That this is a 
controlling tendency may be observed in any closely plicated area. 
The actual folds of a closely folded mass are often like those illus- 
trated in Fig. 55. 

The folds of the zones of fracture and flow therefore contrast in 
the following particulars : 

Zone of Fracture Zone of Flow 

Beds of uniform thickness. Beds thickened and thinned. 

No interior deformation. Interior deformation of all parts. 

Relative competence. Relative incompetence. 

Simple outlines of competent struc- Crenulated and complex outlines of 

ture. incompetent structure. 

Much slipping between beds; dying Little slipping between beds; per- 

out of folds vertically. sistence of folds vertically. 

Folds of above characteristics are Folds of above characteristics are 

"parallel." "similar." 

The use of the terms competent and incompetent respectively 
for the folds of the zones of fracture and flow require some further 
explanation. Willis* experiments on the mechanics of Appala- 
chian structure 1 showed that the thicker, more competent wax 
layers rise in simple outline under given conditions of pressure and 
load until they are unable to lift the load farther. Then they 

1 Willis, Bailey, Mechanics of Appalachian structure: 13th Ann. Rept. U. S. 
Geol. Survey, Pt. 2, 1893, pp. 241-253. 



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112 



STRUCTURAL GEOLOGY 



crumple and, in crumpling, thicken, enabling them to lift the 
load higher. Thus composite folds are really indications of incom- 
petence. Simple folds are more characteristic of the zone of 
fracture; the bed is able to lift itself without interior readjustment, 




Fig. 55. Folded schist from Alaska. Folds are "similar" but the sharpness of the 
bends involves a minimum of distortion of the beds. 

and without crumpling; it is competent. All folds represent a 
yielding to pressure. In that sense all are incompetent, and it 
might be better to speak of them all in terms of degrees of incom- 
petency. There is likely, however, to be little confusion in follow- 
ing Willis in the use of the two terms competent and incompetent. 1 

^p. cit., p. 250. 



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FOLDS OF FRACTURE AND FLOW CONTRASTED 113 

Our field of observation is practically confined to the zone of 
combined fracture and flowage, and hence to folds representing 
some combination of the characteristics of the two zones. The 
folds described as typical of these zones may be regarded as the 
limiting cases between which all folds may be classified. To 
illustrate, interlayered quartzite and slate beds exhibit folds 
characteristic of both zones. The quartzite folds may be of the 
zone of fracture, the shale folds may be of the zone of flow. The 
quartzite layers are in simple, broad, competent folds of the 
" parallel " type developed by fracture without thickening or 
thinning; the intervening slate layers are crenulated, thickened, 
and thinned, relatively incompetent, and of the "similar" type. 
There are many folds in homogeneous beds with characteristics 
intermediate between those described for fracture and flowage 
conditions. 

The principal use of such a classification is not alone to afford a 
means of pigeon-holing various folds, but to call attention to the 
characteristics of folds which it is desirable to know for field study. 
The attempt to analyze a fold in the field and determine what 
combination of fracture and flowage conditions it represents will 
lead to a better understanding of the structure than will the mere 
naming of the fold according to form. For instance, explorations 
for iron ore have been going on extensively in a great slate area, 
completely covered by glacial drift, in central Minnesota. Drilling 
soon demonstrated the fact that the slate was folded in the zone of 
flowage. The observer was therefore justified in concluding that 
the folding was probably close and complex, that there was much 
thickening and thinning of the beds, that the folds were largely of a 
similar type, not dying out above or below. The application of 
these principles, therefore, has been of great aid in interpreting 
fragmentary records brought up from the drill holes, has made it 
possible, for instance, to correlate a thirty-foot bed of ore on the 
Hmb of a fold with a fifty-foot bed near the crest. In the Mar- 
quette district of Michigan, where there are beds of quartzite 
interbedded with softer slates and iron formation, it has been 
possible by the application of these principles to correlate some of 
the simpler and broader structures of the quartzites with the closer, 
much more complex, and quite different folds of the softer beds. 
In making any satisfactory estimate of the thickness of folded beds 



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114 STRUCTURAL GEOLOGY 

the first question to be settled is the degree in which the folds are 
characteristically those of the zone of flowage and therefore to what 
extent they are likely to be thickened or thinned. 

CONTROL OF STRUCTURES IN WEAK BEDS BY DIFFERENTIAL 
MOVEMENTS BETWEEN COMPETENT BEDS ON LIMBS OF 
FOLDS 

Rocks within our field of observation are of varied competence. 
It follows then that in any folded area the structures of the weaker 
rocks are controlled by the folding of the stronger beds. The 
stronger beds tend to assume the "parallel " type of folds in which 
the principal readjustment is between the beds rather than within 
them. This readjustment or slipping is concentrated in the inter- 
vening weaker layers. The structures of the weaker layers indicate 
the direction of this readjustment and thus something of the struc- 
ture of the competent beds. This fact is of great aid in the field 
study and interpretation of a folded area. 

Differential movement between beds is uniformly toward con- 
vex surfaces in the manner indicated in the diagram (Fig. 56). 
In the following pages several criteria will be mentioned by which 
the direction of differential movement may be determined in 
the field. Knowing the direction of such movement, it is possible 
to relate the minor structures to the major folds. 

(1) Minor Folds as Evidence of Differential Movement Between 
Beds. — When areas of heterogeneous rocks are folded the stronger, 
more competent layers are likely to show the characteristics of folds 
of the zone of fracture, and the softer, more incompetent layers to 
show the characteristic folds of the zone of flow, although the two 
kinds of folds may represent neither one extreme nor the other. 
The folds of the weaker layers are really "drag folds" due to 
differential movement between the controlling harder layers. 
The inclination of the axial planes of the minor folds with reference 
to the adjacent competent beds tells the direction of the differen- 
tial movement. The axial planes are nearly parallel to cleavage 
(see pp. 119-120). When this movement is of great proportions 
the axial planes of the minor folds may become so rotated as to 
give the abnormal type of composite fold. Beds of shale may 
indicate a differential movement of quartzite beds above and be- 
low in the direction shown on diagram 56. 



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DRAG FOLDS 



115 



The position of the major fold is inferred from the differential 
movement indicated by the minor folds. The major fold may in 
turn be found to be one of a series of minor folds related to a still 
larger fold. 

This is something more than the statement of an academic 
principle. The writer regards it as one of the most fundamental 
principles in the field study of structures. Adherence to the 
simple plan of watching for indications of differential movement 
leads to surprising results. In the Lake Superior pre-Cambrian 






*v 




Fig. 56. Figure showing differential movement between competent beds on limbs 
of a fold with the development of minor drag folds between them. 

districts it has been possible, by studying the minute crenula- 
tions of the softer beds, to determine the differential movement 
of the controlling strata on each side, and thereby to obtain a 
notion of the position of the next larger unit of structure. This 
has led to a study of still larger units, and so on. In the Mar- 
quette district of Michigan the slate beds are folded in the manner 
to be expected from the control of the harder quartzite layers of 
the Marquette synclinorium. Understanding this relation, the 
composite outlines of the slate folds may be satisfactorily corre- 
lated with the simple outlines of the quartzite folds. The Mar- 
quette synclinorium as a whole may be regarded as a minor fold 
showing differential movement upon the limb of the major Lake 
Superior synclinorium. 



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116 



STRUCTURAL GEOLOGY 




Fig. 57. Photograph of drag fold in sedimentary beds. After Hotchkiss. 

The principle of the control of minor by major folds affords the 
most reasonable hope of working out successfully the complex 
structures of the old Archean or Basement Complex, which hereto- 
fore have been regarded as almost inexplicable. Although on 
casual inspection the folds in any ledge show an apparently great 



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DRAG FOLDS 117 

complexity, when examined with reference to the differential 
movement the general structure becomes more manifest and it is 
possible to infer some of the relations of the major folding. By the 
use of this principle it has been possible recently to work out the 
structure of certain parts of the closely folded Archean of the 
Vermilion district of Minnesota, which have heretofore been des- 
ignated simply as Basement Complex. The writer has observed, 
in traveling over hundreds of miles of Laurentian gneiss, that 
minor folds are accordant over considerable areas, indicating some 
major control and suggesting the possibility of working out larger 
units of structure. 

The "decken" structure of the Alps, illustrated by Fig. 58, 
is a series of great overthrust folds with nearly parallel and hori- 
zontal axial planes, which are probably to be regarded on a large 
scale as minor "drag folds " resulting from the horizontal shearing 
of some formerly existing competent rocks over the Alpine area. 
The great Alpine fan folds of the type §o well known through the 
writings and sections of Heim 1 and others are now being largely 
interpreted by Schardt, Lugeon, 2 and others as " decken " or over- 
thrust folds and faults. The actually observed structures seem to 
permit of connections in cross sections drawn to correspond to 
either hypothesis, and it is probably uncertain in some cases which 
interpretation is the correct one. 

As folds usually have a pitch, the axial lines of minor drag 
folds when projected to the surface uniformly vary a few degrees in 
strike from the strike of the beds at the surface, in all cases where 
the axial lines are not horizontal nor the dips vertical. This is well 
illustrated by folds in iron formation of the Menominee district of 
Michigan (Fig. 59). At one place the iron formation dips 70° 
N. and strikes N. 70° W. The pitch of the minor folds is 30° in a 
direction N. 65° W. As the pitch carries these folds down they are 
carried northward down the dip of the beds. Hence there is a 
divergence of 5 C in this case between the surface projection of the 
axial line of the minor fold and the strike of the bedding, which is a 
fact of some commercial significance in the exploration for ore, in 
view of the fact that the ore follows the pitch rather than the strike. 

1 Heim, Alb., Untersuchunger iiber den Mechanismus der Gebirgsbildung, Basel, 
1878. 

2 See: Der Bail der Schweizerlapen, by Alb. Heim: Neujahrsblatt der Natur- 
forschenden Gesellsehaft in Zurich auf das Jahr 1908, 110 Stuck. 



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118 



STRUCTURAL GEOLOGY 




Fig. 58. To illustrate development of overthrust folding and faulting, accom- 
panied by minor drag folds, as inferred from Alpine structure. After Heim. 



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DRAG FOLDS 



119 



The application of this principle of differential movement not 
only indicates the position of the minor fold with reference to the 
major fold but sometimes affords a means of determining which is 
the top and which the bottom of a bed on the limb of a fold. If 
in an isolated outcrop of vertical beds it is apparent that the left 
hand side has moved up with reference to the right hand side, the 
inference is that the ledge is a part of the left limb of an anticline. 
If so, the left hand beds of the outcrop constitute the top. This 
method has application in the study of isolated outcrops in which 
no other evidence of top or bottom appears. 



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V 



Fig. 59. To illustrate divergence in strike and pitch. After Mead. 

(2) Cleavage as Evidence of Differential Movement in Folding. — 
Fracture cleavage or flow cleavage is usually associated with 
the weaker beds in folds. The attitude of cleavage with reference 
to the bedding indicates the direction of differential movement 
between the beds, and, like the drag fold, becomes an aid in inter- 
preting structure (see Figs. 9-11, 37, 46, 47). When a slate or shale 
is folded between two competent layers, such as quartzite, the 
cleavage produced in the slate affords clear evidence of slipping or 
shearing between the quartzite beds. The cleavage is inclined to 
the bedding at angles determined by the amount of slipping, and 
tends to converge upward on an anticline of gentle curvature. 



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i i:i Brih 



I -I-.. 55. Folded 1 

and without d 
yielding to pre 
might be bettJ 
potency. Thi 

iiiK Willis in thJ 




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CLEAVAGE AND FOLDS 



V21 



The fact that the cleavage is often inclined rather than vertical 
aggests horizontal shearing stresses ratljer than horizontal non- 
ptational compression (see pp. 10-21;; and the**- might l>e d<< 
bloped by the shearing of a large portion J th«- zon«- of Jra<*iur«' 
srer an underlying zone of flow. < 'hamf>erliir ha* *ugge«ted ihat 
brtain great thrust planes mux 1** m*- suria**- expn^ion of a 
Jeep zone of shearing. Shearing movement ha* t^een inougiit by 
Ihamberlin 2 to be a possible re-un o* '-re"p. un'ier gfuvj' y oJ t^* 
evated jx>rtions of th*- earth s '-ru-v e~pe'-jahy a* «oij' n*<-u*;i! 

irgins. Van Hi***" has sugg^<t^j ^a* p*fnaj*« Son: Im'-'ivij, 
fending to retard the surface of *:»* eaT n iv to'aV/j, imi'ii* 
|ve it a tendency to Kiear reia' , \*"*' v^'vaf. o\*~ 
ortions, thereby* giving ea~** vur^.h^jmg '-lea'ajj* . 
ftps of cleavage have n't t>»-*^ *ufh' *j*-i: *•; v.* 



,rge areas to a^-enain to v:;u~ **x*.**ir t.v-* v„iy;.i* *,*/*a ;-j/<,j,<. 



a — 



/ tj' 






nth the requireiii^iiv of ary on* o *:j*-^* ;*• jrrri*--*-- 
(3) Jointing. f'-ur.u"~f \*n <i<y "><' i ^ ' *. *.s h<<»u>, 

^iffererdt-al Mowin^n* h^.'^^i h>* * / / j.<-i>* t ';Am< 

aovement betweex beu- u*-".*^ »:♦* on* ~' o --,-,•«. -^- j, w;** 5 ;,,, 
i the beds and an'rn^ a* ai a:»; f .' e ^ ** ^ ^ ' v t i> J ,. 

The latter net inih'/av^ ~:i* r ::**~"i /i / * >* ',,- , ,, *-n* * •-- 
Is in folding. «r>nr> or .v ;** - >^ - -^* , ,-<; ,-, / y 
Iplane?- Tney ma; **- *-u~.*-'. «r *•--*. <> > .^ > ,- ,. . 
[be confined 10 »^.uii * 
fin passing v.- :i:n**r*^r -*- 

thu* obviou-:;* r^a'e' v 

differed* ibi niT'eu>*ir a:. r 3**- 1 :, / r. 

on wider o*>-*erva~j Hi- ^ *-: / r • ,- 

trict of Wiv«jii-a- ur* »- .-" ' • y 

by tiro **et* r r / ».*::•- *.*i* i*»- ;^-- , .• 

set off ^"uT.i^e j n:r*- **t >■**.. ^ * >* >*-. 

(seeTyst. 11 un: -- i * ,-,- .*, . • 

liave ui f ,»vec >j'J : ; * v u* * r ; .* *-.-:*-■- ^ 

respoBUOt v. '.n^ r*r ^**^^-*. , + . ^ y 

S\"ttelrLie. Tie -uiu- :-',' .- ;<k--.' - . - 

limb 'C ZTjh hara / * - „ • > -^ 

x Xvl35j~ ' J- . .^^-^ . . 



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120 STRUCTURAL GEOLOGY 

The cleavage is approximately parallel to the axial planes of minor 
drag folds which are likely to be present under such conditions 
(see p. 114). 

Some beds are so closely compressed that both the cleavage 
and the bedding are so nearly vertical as to be about parallel. 
Here it may be impossible to apply the above simple principles 
of relationship; but the writer has found that when a detailed 
study has been made it has sometimes been possible even here to 
draw some reasonable inference as to the position of the axial 
planes of the closely compressed folds. In irregular anticlinoria 
or synclinoria the cleavage may apparently have such intricate 
relations to bedding that it is impossible to formulate any general 
statement of the relations of cleavage to the fold as a whole. 
Examination of any detail of the fold, however, will indicate the 
relations above described, and from these details much light may 
be thrown upon the general character of the fold. 

When all parts of a homogeneous incompetent rock are folded in 
the zone of rock flowage, there is a less pronounced shearing be- 
tween beds, and less control of cleavage directions by differential 
movements on the limbs. The cleavage may have a uniform dip 
regardless of folds, but in general is parallel to the axial planes. 
Both the cleavage and the folds may then be regarded as having 
been developed under some larger control, as, for instance, the 
shearing of a rigid mass horizontally over the entire area. This is 
illustrated by monoclinal cleavage crossing the complex folds in 
slate without any apparent relation to the minor folds; whereas 
when compared with major folds in adjacent competent strata the 
cleavage is found to be in positions which indicate its development 
under the control of a major fold. The same principle on a larger 
scale may be considered as explaining some of the regional cleavage. 

Large areas like the Piedmont Plateau and parts of the pre- 
Cambrian shield of North America have a cleavage with remark- 
ably uniform strike and dip, notwithstanding the heterogeneity of 
rocks and folds on these areas. There seems to have been some one 
factor controlling the development of cleavage for the area as a 
whole. There is no reason to believe that the development of such 
cleavage does not conform to the laws of stress and strain already 
described, but the units of structure involved may be much 
larger than those observable in the individual folds. 



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CLEAVAGE AND FOLDS 121 

The fact that the cleavage is often inclined rather than vertical 
suggests horizontal shearing stresses rather than horizontal non- 
rotational compression (see pp. 16-21); and these might be de- 
veloped by the shearing of a large portion of the zone of fracture 
over an underlying zone of flow. Chamberlin 1 has suggested that 
certain great thrust planes may be the surface expression of a 
deep zone of shearing. Shearing movement has been thought by 
Chamberlin 2 to be a possible result of creep, under gravity, of the 
elevated portions of the earth's crust, especially at continental 
margins. Van Hise 3 has suggested that perhaps tidal friction, 
tending to retard the surface of the earth in its rotation, might 
give it a tendency to shear relatively westward over underlying 
portions, thereby giving eastward-dipping cleavage. Strikes and 
dips of cleavage have not been sufficiently well correlated over 
large areas to ascertain to what extent they might correspond 
with the requirements of any one of these hypotheses. 

(3) Jointing, Fracture-Cleavage, and Fissility as Evidences of 
Differential Movement Between Beds in Folding. — Differential 
movement between beds develops one set of shearing planes parallel 
to the beds and another at an angle less than 90° to it (see Fig. 7). 
The latter set indicates the direction of the displacement between 
beds in folding. Joints or fracture cleavage form along these 
planes. They may be curved or S-shape. Also they are likely to 
be confined to certain beds and offset along the bedding plane 
in passing to different strata on either side. Given, then, joints 
thus obviously related to folding, it is possible to determine the 
differential movement and get a notion as to the part of the fold 
on which observation is taken. For instance, in the Baraboo dis- 
trict of Wisconsin, northward dipping beds of quartzite are cut 
by two sets of joints, one set parallel to the bedding, and another 
set of strike joints crossing the bedding and dipping northward 
(see Figs. 10 and 11). It is clear in this instance that the upper beds 
have moved southward with reference to the lower beds. This cor- 
responds to the requirements of a position on the south limb of a 
syncline. The same kind of reasoning may be applied to the north 
limb of the Baraboo syncline (see Fig. 9). 

1 Chamberlin, T. C. f The fault problem: Econ. Geol., Vol. 2, 1907, p. 598. 

2 Idem., p. 718. 

3 Van Hise, C. R., A treatise on metamorphism : Mon. 47, U. S. Geol. Survey, 
1904, p. 930. 



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122 



STRUCTURAL GEOLOGY 




Fio. 60. Illustrating the artificial development of fold. After Willis. The fold 
begins to develop at points of initial irregularity in the beds (initial dip) near 
the point of application of force. The heavy layers rise in simple, com- 
petent, parallel folds, the soft layers in composite, incompetent, similar folds. 
When the stronger layers have risen to the limit of their competency they 
buckle, developing composite outlines and to that extent taking on character- 
istics of incompetent folds. 

It is frequently necessary to interpret structure from a few 
widely separated exposures, and then relations of this type may 
furnish a clue to the structure, to be checked of course by other 
criteria. 



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DIFFERENTIAL MOVEMENTS IN FOLDS 123 




Fig. 60 (continued.) 

Conclusion as to differential movements in deformation. Our field 
observation being confined to the zone of combined fracture and 
flowage where the beds are both competent and incompetent, 
it follows that the larger part of the rock structures and the 
rock deformation described in this book may be regarded as 
evidences and results of differential movement on a smaller or 
larger scale. 



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124 STRUCTURAL GEOLOGY 



LOCALIZATION OF FOLDS 



(a) It has been shown experimentally by Willis 1 that folds tend 
to form near the point of application of the deforming force, unless 
the rocks are sufficiently rigid to transmit the thrust forward to 
some weaker zone, (b) Willis has also shown that slight irregular- 
ity in the bedding, such as might be formed during sedimenta- 
tion, and which he calls initial dip, will tend to localize a fold, even 
at some distance from the point of application of the stresses, 
(c) Still further, it appears that the uplift of the fold at any one 
place may tend to depress the beds immediately beyond it, creating 
an irregularity or " initial dip " which localizes another fold. The 
first fold rises to such a point that it becomes easier to develop a 
new fold than to lift the old fold higher, (d) Irregularities of 
structures other than bedding may localize the fold. Contact of 
rocks of unequal strength, for instance of granite and sediments, 
has been observed to localize folds, the massive granite serving as a 
buttress against which the weaker series is deformed, (e) Inherent 
weakness of rocks may localize a fold. A slate is likely to be more 
folded than an adjacent quartzite. Initial dip or other irregularity 
may determine at what points in the shale the folds shall be 
localized, but the weakness of the shale as a whole as compared 
with the adjacent beds will favor the development of folds in the 
shale rather than in the quartzite. This weakness is one of the 
common causes for the localization of folds. 

DETERMINATION OF DEPTH AFFECTED BY FOLDS 

It is sometimes possible to measure the linear shortening of an 
area by folding, and also the vertical uplift. These are necessary 
data for estimating the depth affected by the folding. In the 
following diagram, which may be supposed to illustrate roughly 
the Southern Appalachian folding, 100 miles of surface has been 
crowded into 75 miles. There has been an uplift of approximately 
a mile. Obviously the product of the linear uplift and the length 
of the shortened area, 1 mile x 75 miles, should equal the product 
of the shortening, 25 miles, and the depth affected. By solving the 
equation, this depth is found to be 3 miles. This method was 

1 Willis, Bailey, Mechanics of Appalachian Structure: 13th Ann. Rept. U. S. 
Geol. Survey, pt. 2, 1893, p. 247. 



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DEPTH OF FOLDS 



125 



suggested by T. C. Chamberlin l and has been applied by R. T. 
Chamberlin 2 to the Appalachian folds of central Pennsylvania. 
A similar method was independently developed by Willis 3 and 
applied to the Cascade Mountains. The same method has been 
suggested by T. C. Chamberlin 4 to determine depth affected by 
faults. 

With a given elevation, the less close the folding (or faulting) 
and therefore the less the shortening, the greater the vertical 
distance involved in the deformation. In the section made by 



-* Present fertgi 



Present length of folded section 



Average uplift — * 

t 



Shortening 6/ folding* 



« 

I 



Deforming fore* 



Fig. 61. Illustrating a method of determining depth affected by folds. 

R. T. Chamberlin 5 from Harrisburg to Tyrone in Pennsylvania 
he finds that shallower depths are affected on the two ends of 
the section and greater depths toward the center (see Fig. 62). 
The shallowest deformation found is 5.7 miles. Making calcula- 
tions for five sub-sections, he finds a gradual increase in the depth 
affected toward the center of his section, which suggests that the 
deformed zone is bounded by planes dipping approximately 45° 
from the surface at either end of the section and intersecting about 
32 miles below the surface near the center. The intersection of 
these hypothetical planes at 45° with each other and with the 
earth's surface suggests to Chamberlin that they are really shearing 

1 Chamberlin, T. C., and Salisbury, R. D M Geology, Vol. II, 1906, pp. 126-126. 

2 Chamberlin, R. T., Appalachian folds of central Pennsylvania; Jour. Geol., 
Vol. 18, 1910, pp. 228-251. 

3 Willis, Bailej', Physiography and deformation of the Wenatchee-Chelan dis- 
trict, Cascade Range: Prof. Paper No. 19, U. S. Geol. Survey, 1903, pp. 95-97. 

4 Chamberlin, T. C, The fault problem: Econ. Geol., Vol. II, 1907, p. 596. 

5 Op. cit., p. 245. 



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126 



STRUCTURAL GEOLOGY 



i 

2 




i sis 
Jffl- 3 



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ft = _£ 5 



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DEPTH OF FOLDS 127 

planes developed by tangential shortening in the manner of frac- 
ture planes formed in a block under pressure. 

The above inference implies that the pressure has been applied 
with equal intensity on all unit areas on the sides of the deformed 
block; it implies a non-rotational strain; it implies, further, that 
shearing planes find expression in the zone of rock flowage. While 
shearing stresses are undoubtedly present in this zone during 
deformation, it is not so clear that they would find expression in 
definite planes bounding the deformed region or that such planes 
would have the position assumed for them. They would not if the 
strain were rotational, developed by tangential stresses. The 
structure consonant with such deformation in the zone of flowage 
is a vertical cleavage, as is implied by Willis' conclusion concerning 
the Cascade folding. 1 The depth reached by the deforming move- 
ments of the Cascade uplift has been calculated by Willis to be 
from 375 to 1500 miles. The smaller of these estimates would lead 
so deep into the zone of flowage as to make it impossible to con- 
sider the deformation as being controlled by shear zones. Willis 
seems to have considered that the entire mass has been shortened 
by flowage down to these depths, resulting in vertical uplift. 

The methods for the determination of depths of folding worked 
out by the Chamberlins are of fundamental significance, and are 
likely to yield unexpected results. 

Another way of estimating the depth affected by folding is to 
compare the deformation in stratigraphically superposed rocks in a 
given locality. Daly 2 finds in south-central British Columbia that 
the pre-Cambrian massives are much less folded than the overlying 
Carboniferous and Triassic rocks, indicating that a small depth of 
the earth shell has suffered strong folding in post-Cambrian time. 

FIELD OBSERVATIONS ON FOLDS 

Strike and Dip: — Strike and dip records are ordinarily of value 
because of the light they throw on the folding of strata. It is 
essential in taking the readings to keep this in mind in selecting 
points at which to take the observations. Especially it is desirable, 

1 Willis, Bailey, Physiography and deformation of the Wenatchee-Chelan dis- 
trict, Cascade Range: Prof. Paper No. 19, U. S. Geol. Survey, 1903, pp. 92-97. 

2 Daly, R. A., Abstract of paper presented at 24th annual meeting of Geol. Soc. 
Am. at Washington, D. C, December, 1911. 



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128 STRUCTURAL GEOLOGY 

as soon as the existence of a fold is suspected, to search for the 
axis, in order to ascertain the pitch. In a closely folded area the 
deformation of the beds by shearing on the limbs is so much greater 
than on the axial lines that frequently much can be ascertained 
from a study of the axial zone which could not be suspected from a 
study of the limbs alone. An illustration may be cited from the 
folded Algonkian and Archean rocks in the Vermilion district of 
Minnesota. The Archean is exposed in the cores of closely folded 
anticlines. Along the sides of these anticlines the shearing is so 
close that cleavage has been developed both in Archean and Algon- 
kian, and the evidence of their relations practically destroyed. 
On the axis of the fold, however, where it pitches under the sur- 
face, it is frequently possible to find the beds so little deformed 
that conglomerates may be recognized and the relations worked 
out. 

The determination of the pitch of the axis gives the dip of the 
limb of the cross fold. 

The taking of strike and dip observations at random without 
a definite attempt to correlate them on to the general structure 
of the district at the time they are taken leads frequently to 
unsatisfactory results. Daily field study of strike and dip observa- 
tions, conscientiously platted to date, should be the basis for 
planning field work on succeeding days. Too frequently, definite 
field determinations of pitch are not made, but are left to be in- 
ferred from a study of the records when later platted. Thus 
one of the most important and decisive elements of structure is 
loosely determined, and this neglect may often lead to serious error. 

Emphasis on Relations of Major and Minor Structures: — The con- 
stant attempt to correlate minor and major structures under the 
principles outlined in the preceding sections cannot be too strongly 
urged. It is indeed surprising what a variety of applications these 
principles have. It is seldom that a study of any element of the 
structure does not give a clue as to what to expect in the larger or 
smaller elements of the deformation. 

Field Observations on Relations of Cleavage to Folds: — Keeping in 
mind the simple relationships of cleavage to folds, discussed on 
pages 119-121, the following are some of the field inferences that 
may be drawn from cleavage. The student will find it to his 
advantage to reason out each of these inferences for himself. 



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FOLDS AND CLEAVAGE 



129 



(a) Cleavage converging upward suggests an anticline. It is 
seldom, however, that this ideal condition may be recognized in 
the field on any large scale. The slight overturning of cleavage or 
folding makes it difficult to determine this relation, (b) More 
useful are the inferences to be drawn perhaps from local observa- 
tions of the relation of cleavage to bedding. Cleavage normal to 
bedding probably indicates the axial plane of the fold, (c) Cleav- 
age inclined to the bedding probably indicates the limb of a fold, 
(d) The inclination of the cleavage with reference to the bedding 
tells on which limb of the fold the observation is taken, (e) If 
bedding is vertical and inclined cleavage is present in the softer 




•;:•;:■>::*.:. .Potsdam 
Shale bed i-sssss; 






'W//////////A 



Fig. 63. Vertical section of Illinois mine, Baraboo district, Wisconsin. After 

Weidman. 



layers between harder ones, thereby indicating direction of dis- 
placement, it is possible to infer on what part of the fold this 
relation was doubtless developed and from this in turn it may be 
inferred which is the top and which the bottom of the bed. 

In the Baraboo district of Wisconsin, slate overlain by iron 
formation has been folded between a competent quartzite layer 
below and a dolomite bed above. The slate thus forms the foot- 
wall for the iron formation. A shaft sunk largely in the slate 
followed the cleavage, the bedding being very obscure. As a 
result, the shaft penetrated the ground more steeply than the 
bedding, as would be expected, and where at considerable depth 



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130 STRUCTURAL GEOLOGY 

drifting was begun to cross cut the ore, it was found that the 
bottom of the shaft was a long distance away from the ore body. 

The greater part of the Lake Superior iron ore is found in the 
upper Huronian group of rocks, of which slate is an important 
member. Much of the exploration has to be done by drilling, 
and a study of the relations of cleavage to bedding in the slate 
brought up in the drill cores frequently gives important clues 
to the folding. For instance (a) a vertical drill hole discloses a 
vertical cleavage with a horizontal bedding. The inference is 
that the hole is parallel to the axial plane of the fold, (b) It dis- 
closes cleavage inclined to the bedding. The inference is that the 
limb of the fold has been penetrated, (c) A hole drilled at an angle 
of 45° to the horizon brings up a core, the longer direction of which 
bisects the acute angle between the cleavage and bedding. The 
general trend of the principal elements of structure of the district 
is known. It is not known, when the core is brought up, how much 
it has been rotated in the hole, and thus from the core two hy- 
potheses are possible — that the bedding is nearly horizontal and 
the cleavage nearly vertical, or that the bedding is vertical and the 
cleavage horizontal. The fact that cleavage in these slates is 
usually vertical or nearly so makes it necessary in the majority of 
cases to conclude that the bedding is horizontal, and that it has 
been cut near the axis of a fold. 

Many other specific illustrations might be given to show the 
value of this principle in field work. If the observer of drag folds 
or cleavage will in every case ask himself what is the displacement 
shown by these structures taken in detail or as a whole, he will be 
able to determine his probable position with reference to the 
next larger order of fold, and hence to direct his work more intelli- 
gently in working out the features of this larger element of struc- 
ture. 

If cleavage alone is observed, with unknown relations to bed- 
ding, some valuable inferences are still possible. The very exist- 
ence of cleavage implies failure or incompetence on the part of the 
rock and this in most cases involves folding. The writer has yet 
to find a true slate which does not have some folding of the beds. 
It may be inferred also that the folds are characteristic of flowage 
conditions, that is, similar folds with composite outlines, and that 
their axial planes are nearly parallel to the cleavage. This incom- 



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RIPPLE MARKS 



131 




Fig. 64. Photograph of (a) ripple marks and (b) casts of ripple marks. 
After Van Hise. 



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132 STRUCTURAL GEOLOGY 

petent structure is almost certainly controlled by competent struc- 
tures in stronger adjacent rocks wherever they may be. The pre- 
vailing strike and dip of cleavage may suggest where and what the 
larger competent structure is. Cleavage in a slate area may 
strike east and west and dip south at an angle of 45°. The inference 
is that here are similar composite folds with east-west trend and 
axial planes dipping to south; further, that the structure was 
developed by the relatively northward movement of some over- 
lying competent rocks which have been removed; finally this 
inferred major control suggests a major anticline to the north. 
Determination of Top and Bottom of Sedimentary Beds in a Folded 
A rea: — It is only in folded beds that criteria other than super- 
position are necessary to determine top or bottom of the beds. 
When the folding is worked out the problem is solved. Any 
methods used for determining folds therefore apply to this prob- 
lem. The relations of cleavage, joints and minor drag folds 
to major structure discussed above therefore help to determine 
which is top and which is bottom of beds. There are primary 
structures of beds which may also be used to advantage, par- 
ticularly (a) ripple marks, (b) false bedding and (c) variations in 
coarseness of grain. 

(a) In Fig. 64 the normal ripple marks and their casts are indi- 
cated. It will be noted that in the normal ripple marks the crests 
are much sharper than the troughs, and that the troughs may have 
minor crests in them. When the beds are on edge or overturned, 
these facts enable one to tell which is top and which is bottom. 

(b) In Fig. 65 it will be noted that the false bedding is abruptly 
cut off by overlying beds while it comes in contact with the lower 
beds by a tangential curve. If the outcrop shown in the photo- 
graph were turned on edge or overturned, there would still be 
no difficulty in determining which were the original top and bottom 

of the beds. 

(c) It is very common to find a diminution in coarseness of 
beds from the bottom toward the top. Even in microscopic sec- 
tions this is apparent. The beds may start in abruptly with 
coarse sediments, these gradually become finer-grained above, 
and the next bed start in again abruptly with coarser sediments. 
There is little difficulty in these cases, no matter what the folding, 
in determining the original top and bottom of the beds. This has 



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CROSS BEDDING 



133 




Fig. 65. False bedding or cross bedding in sandstone. Dalles of the Wisconsin. 
After Salisbury and At wood. 



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134 STRUCTURAL GEOLOGY 

been found especially useful in interpreting drill samples from 
folded rocks. 

SUGGESTIONS FOR LABORATORY STUDY OF FOLDS 

The following questions merely suggest a desirable line of laboratory 
study. Teachers will multiply illustrations. 

On the Cloud Peak-Fort McKinney, Wyoming, or Oelrichs, South 
Dakota-Nebraska, folios (Nos. 142 and 85, U. S. Geol. Survey), how can 
the strike of the beds be determined from the geologic map? Show also 
how the direction and approximate angle of the dip can be found from the 
map. On the Monterey, Virginia-West Virginia, or Ringgold, Georgia- 
Tennessee, folios (Nos. 61 and 2, U. S. Geol. Survey), determine the direc- 
tion and degree of pitch of axial lines of both anticlines and synclines. 
Are the strike and pitch parallel? What are the various possible relations 
between them? What do these relations signify? Study the valleys and 
outcrops on the Sundance, Wyoming-South Dakota, folio (No. 127, U. S. 
Geol. Survey) using the geologic map. How do the shapes of outcrops 
vary with different relationships between the dip of the beds and the 
direction and gradient of the valleys? 

On the Monterey, Virginia-West Virginia, or Three Forks, Montana, 
folios (Nos. 61 and 24, U. S. Geol. Survey) show how anticlines may be 
distinguished from synclines by the study of outcrops on the geologic 
map; the same with reference to anticlinoria and synclinoria on the Mt. 
Mitchell, North Carolina, and Menominee, Michigan, maps (folios 
Nos. 124 and 62, U. S. Geol. Survey). 

On the geologic maps of the Mt. Mitchell, North Carolina, folio (No. 
124, U. S. Geol. Survey) show how the outcrops themselves indicate that 
certain folds are overturned; that some of the folds are isoclinal. 

On the Maynardville, Tennessee, Bristol, Virginia-Tennessee, and 
Morristown, Tennessee, folios (Nos. 75, 59, and 27, U. S. Geol. Survey) 
study the relation of the little drag folds to the major folds of the region. 
Is there any relation between the drag folds and certain rock formations? 
Why? What is the relation between the pitch of the drag folds and that 
of the major folds? What differential movements do the drag folds in- 
dicate and where and of what type are the major folds? 

Examine cross sections on the Maynardville, Tennessee, and Morris- 
town, Tennessee, and Mt. Mitchell, North Carolina, folios (Nos. 75, 27, 
and 124, U. S. Geol. Survey) and the Marquette, Michigan, monograph 
of the U. S. Geological Survey (Vol. 28). Are the synclinoria normal or 
abnormal? What caused the one type to be developed rather than the 
other? 

Are the folds on these cross sections similar or parallel or are both types 
present? 

Which of the folds studied on foregoing maps were formed in the zone 
of rock flow and which in the zone of fracture? Why? 



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LABORATORY STUDY OF FOLDS 135 

Given an outcrop of steeply inclined beds, what are the various phenom- 
ena to be looked for indicating differential movement between the beds? 
(See Figs. 9, 10, 11, 37, 46 and 47.) 

Having determined the direction of the differential movement, what 
inference do you draw as to type of folding, as to location and pitch of the 
axes of the major folds, as to the top and bottom of the beds? 

Study the charts accompanying Willis' "Mechanics of Appalachian 
Structure," x with a view to answering the following questions: What has 
determined the location of the folds? How are folds repeated? What 
determines whether the fold shall be simple or composite in outline? 
Which of the folds are of the abnormal type and why have these developed? 
Are the folds similar or parallel? Are they characteristic of the zone of 
rock fracture or the zone of rock flow? 

1 Willis, Bailey, Mechanics of Appalachian Structure: 13th Ann. Rept. U. S. Geol. 
Survey, pt. 2, 1893, pp. 211-281. 



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MOUNTAINS 

TYPES OF MOUNTAINS 

Mountains may be carved by erosion from undeformed sedi- 
ments or undeformed igneous rocks. They may be formed en- 
tirely by volcanic extrusion without the aid of erosion or secondary 
deformation. The larger mountain ranges are sculptured in rocks 
which have undergone secondary deformation and uplift. They 
are commonly dated from the time of deformation and uplift, 
rather than from the period of erosion. Depending on the nature 
of the deformation, they are called block fault mountains, mono- 
clinal fold mountains, fan fold mountains, etc., though it has been 
recognized that erosion has been an important factor in causing 
the present topography. Uplift relative to sea level must precede 
erosion and in that sense is primary and essential to mountain 
building. The uplift, however, may produce a plateau or other 
forms quite different from mountains. Differential erosion there- 
fore is necessary to produce the forms of mountains. In time 
erosion completely base-levels mountains, as it has so largely in 
pre-Cambrian areas. In the highest existing mountains the up- 
lift and deformation have been of recent date and erosion has not 
had time to reduce them. 

The structure of the greater number of mountains is clearly 
the result of tangential shortening of the earth's crust expressed 
in folding and overthrust faulting. They exist in chains of elon- 
gated ridges, lying end to end, or overlapping. They afford 
marked evidence of greater shortening normal to the general trend 
of the chain than parallel to it. Certain of the folds show an 
irregular dome-shape and seem to have been shortened more or less 
equally from all sides. 

Attention is here especially directed to mountains developed by 
differential erosion of rocks which have undergone secondary 
deformation. They are conspicuous surface expressions of the 
structures described in the earlier pages of this book and this 
discussion will therefore be brief. 

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MOUNTAINS AND FAULTS 137 

MOUNTAINS AND NORMAL FAULTS 

Several mountain ranges are the result dominantly of nearly 
vertical movements along faults. Examples of these are found 
among the Great Basin ranges of the West where there are faults 
with a displacement of over a mile and in the Wasatch and Sierra 
Nevada mountains. The present topography of the Great Basin 
ranges is due partly to fault scarps, more or less modified by ero- 
sion. There is a difference of opinion among geologists as to the 
relative importance of the two factors of faulting and erosion. 
The published discussion of the subject is of general interest, as 
illustrating the trend from an earlier emphasis on structural fea- 
tures, such as faults, toward a wider recognition of the importance 
of erosion. (See pp. 57-59.) 

MOUNTAINS AND THRUST FAULTS 

While thrust faulting has played an important part in the 
deformation of many mountain ranges and the faults influence the 
present topography, erosion has so modified the fault topography 
that it is difficult to state in simple terms the influence of faulting 
in producing this topography. In general fault slices piled one on 
top of another tend to form the present elevations. This is con- 
spicuously illustrated in the Highlands of Scotland, in the Scan- 
dinavian Highlands, in parts of the Alps, and in the southern 
Appalachians. Erosion, working on the tilted fault slices, leaves 
linear ridges generally, but not closely, parallel to the fault traces, 
but the varying hardness of the rocks and the physiographic 
conditions play such an important part that there is usually no 
close relation between the mountain range and the fault traces. 
Such ridges tend to be steeper on the side toward which the over- 
thrust is moving and gentler on the other side. The fault traces 
naturally are exposed on the steep faces. 

MOUNTAINS AND FOLDS 

Where the folds are somewhat simple and open there is a dis- 
tinct tendency for erosion to cut down the anticlines, leaving the 
synclines as ridges between. Synclinal mountains thus formed are 
well illustrated in the Appalachian region. The stumps of moun- 
tains throughout the pre-Cambrian are largely of this type. The 



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138 STRUCTURAL GEOLOGY 

iron " ranges M of Lake Superior, which are really stumps of moun- 
tains, are prevailingly synclinal. Less frequently the anticline 
stands as a topographic elevation. Illustrations of this may be 
seen in the Appalachian mountains. This structure is more com- 
mon where the anticline has a core of igneous rock, as in the Front 
Range of Colorado. 

As the folding becomes closer and more complicated, the rela- 
tions to topography likewise are more complicated. The great 
overthrust folds or "decken" structure of the Alps and to a less 
extent some of those of the southeastern Appalachians bordering 
the Piedmont, illustrate this complexity of relations. The general 
effect is to pile up strata in the same manner as in overthrust 
faults, forming ridges, which in general mark the present elevations, 
but the varying resistance of the rocks to erosion from various 
causes results in wide variations in topography. As in the case 
of thrust faults, the steep slopes tend to be on the side away from 
the thrust; in gentle slopes, toward the thrust. 

In an area of monoclinal folding the softer beds are eroded and 
the more resistant beds stand out as linear ridges with steep sides 
generally in the direction opposite to the dip and with gentler 
slopes in the direction of the dip. Somewhat regular step moun- 
tains or step topography may be produced in this fashion. 

MORE COMPLEX RELATIONS OF MOUNTAINS TO 

STRUCTURE 

The above statements express but crudely some of the simpler 
relations between structure and mountain ranges. In most moun- 
tain ranges there have been repeated deformations and uplifts and 
repeated cycles of erosion which leave the present topography in 
relations to structure which cannot be as simply stated as above. 
It has often been possible to work out the complex history of the 
relations between structure and erosion in the development of the 
present topography, but this has been primarily the field of the 
physiographer and will not be entered into here. 

LOCALIZATION OF MOUNTAINS 

Mountains due to deformation are located where folding and 
faulting accompanied by uplift result from failure of the earth's 



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LOCALIZATION OF MOUNTAINS 139 

shell. The part of differential erosion is in one sense secondary 
and modifying. The highest mountain chains are those of recent 
age, which erosion has not yet had time to cut down. The older 
rocks, principally the pre-Cambrian, show deeply eroded, folded, 
and faulted stumps of mountains. Suess has emphasized the 
extreme deformation of the pre-Cambrian rocks and implies that 
the Archean was a greater mountain-building era than any era 
since. It is apparent, however, that the Archean has suffered 
deformation not only during Archean time but during all succes- 
sive periods. Consequently it shows in general more folding than 
the rocks of later periods, but it does not follow that this excessive 
amount of deformation was accomplished during the pre-Cambrian, 
rather than during later periods. While it is entirely conceivable 
that the Archean may have been a time of mountain building on a 
far greater scale than any succeeding period, the writer doubts 
whether this has been established on an inductive basis. 

Mountain areas of earlier periods have commonly been the locus 
of mountain building in later periods. Some zones of weakness 
seem to have been permanent through much of geologic history. 
Many of the principal mountain chains are the result of repeated 
foldings and uplifts along the same general zone. There has been a 
tendency also for successive deformations to widen the moun- 
tainous zone. 

Many, in fact most, of the great mountain chains are near the 
margin of continents. Some mountain chains which do not now 
border continents did so at the time of their deformations. It has 
long been recognized that mountains have developed at various 
periods in geologic history along geosynclinal shores of heavy dep- 
osition. Thus the Appalachian mountains developed along the 
shore area of heaviest deposition of the Paleozoic sediments against 
the old pre-Cambrian Appalachia, now represented in part by the 
Piedmont plateau. 

The distribution of mountain chains along continental margins 
suggests crowding between oceanic and continental segments 
of the globe. Chamberlin 1 considers such crowding to be due to 
the settling of the larger and more dense oceanic segments as a 
whole, crowding smaller and less dense continental segments 
laterally and possibly upward, and localizing deformation near 

1 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. 1, 1904, p. 521. 



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140 STRUCTURAL GEOLOGY 

continental margins. According to the advocates of the theory of 
isostasy, this crowding is due to the readjustments necessary to 
restore equilibrium between regions of different density when this 
equilibrium has l>een disturbed by transfers of material by erosion, 
or by any other agency. By others the localization of mountains 
in these zones has been referred more or less vaguely to a rise of 
the isogeotherms into the base of the thick mass of sediments 
deposited in a geosyncline, softening and weakening them, and 
thereby localizing deformation by general earth stresses, whatever 
their origin. The causes of earth movements are discussed in a 
subsequent section. The foregoing is merely to notice the localiza- 
tion of mountains by crowding near continental margins. 

SUGGESTIONS FOR LABORATORY STUDY OF 
MOUNTAINS 

To what extent may the topography be said to be dominantly influenced 
by folding or faulting or other secondary rock structures in the following 
areas : 

In the southern Appalachians: See U. S. Geological Survey folios, es- 
pecially Monterey, Va. (folio No. 61), Cranberry, N. C. (folio No. 90), 
and Rome, Ga. (folio No. 78). 

In the Alps: See Mechanismus der Gebirgsbildung, by Albert Heim, 
1878, and Geologische Probleme des Alpengebirges, by G. Steinmann: 
Zeitschrift des Deutschen und Osterreichischen Alpenvereins, Vol. 37, 
1906. 

In the Highlands of Scotland: See The geological structure of the north- 
west Highlands of Scotland, Mem. Geol. Survey, Great Britain, 1907. 

In the Great Basin region: See origin and structure of the Basin Ranges, 
by J. E. Spurr: Bull. Geol. Soc. Am., Vol. 12, 1901, pp. 217-270; also 
U. S. Geological Survey folios on this region. 

In the Rocky Mountains: See stratigraphy and structure, Lewis and 
Livingston Ranges, Montana, by Bailey Willis: Bull. Geol. Soc. Am., 
Vol. 13, 1902, pp. 305-352, and the following U. S. Geological Survey 
folios: Spanish Peaks, Colo, (folio No. 71), Sundance, Wyo. (folio No. 127), 
Three Forks, Montana (folio No. 24), Livingston, Montana (folio No. 1), 
Little Belt Mountains, Montana (folio No. 56). 

In the Ozarks: Tahlequah, Ind. Terr., geologic folio (No. 122). 



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MAJOR UNITS OF STRUCTURE 

Geanticlines, Geosynclines, Ocean Basins, Continents, Plateaus, 
Positive and Negative Elements 

In addition to the secondary rock structures and their expres- 
sions in mountains, discussed in previous pages, we have to con- 
sider certain larger secondary earth structures not ordinarily 
within the range of our detailed observation or mapping. These 
are continents, plateaus, ocean basins, geanticlines, geosynclines, 
positive and negative elements. 

The major units of structure of the kind indicated in the 
above heading require no definition, with the possible exception of 
geanticlines and geosynclines, and positive and negative elements. 
Geanticlines are merely anticlines affecting a large area. They 
differ only in size from anticlines, and the delimiting size is indef- 
inite. Willis 1 has used the name "positive element " for portions 
of the earth's crust which have tended during geological time to 
rise and thereby remain uncovered by marine sediments, as con- 
trasted with " negative elements" which have been submerged 
again and again during geologic history. The pre-Cambrian shield 
of North America is a positive unit; the Paleozic area of the Missis- 
sippi Valley is a negative element. These divisions are necessarily 
vague and their boundaries have shifted widely during geologic 
time. 

SHAPES OF MAJOR ELEMENTS OF STRUCTURE 

A notable expression of the common tendency toward generaliza- 
tions from complex facts is the frequent attempt of geologists to 
read into the lineaments of the earth's surface patterns correspond- 
ing to hypotheses of the origin of the earth or earth deformation. 
One of the best known early attempts at this was the so-called 

1 Willis, B., A theory of continental structure applied to North America: Bull. 
Geol. Soc. Am., Vol. 18, 1907, p. 390. 

141 



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142 STRUCTURAL GEOLOGY 

tetrahedral theory of the earth. A tetrahedron is a solid body 
which possesses the greatest possible surface for a given volume. 
On the hypothesis that the earth's interior is molten and is cooling 
more rapidly than its shell, it was assumed that the shell would 
tend to maintain the largest possible area of surface and therefore 
might take on tetrahedral lineaments. Continental areas and 
mountain chains would then correspond roughly to the angles and 
corners of the tetrahedron. By standing a tetrahedron on one of 
its corners and calling this point the south pole, the three upper 
corners and angles are supposed to correspond to the land areas 
surrounding the north pole. The three angles extending down 
toward the south polar point would correspond to the continental 
ridges of South America, Africa, and Australasia. The dominance 
of the land area in the northern half of the continent would accord 
with the dominance of projections in the upper half of the tetra- 
hedron. It is needless to say that this comparison requires some 
imagination. It is cited merely as illustrative of the several 
hypotheses offered. Equally good comparisons have been made 
with other geometric forms. 

A more recent generalization is that of Chamberlin, 1 who sug- 
gests that the great negative elements of the earth, represented 
largely by sea areas — the master segments — should be expected 
to have polygonal outlines corresponding to the primary place 
assigned them; that the smaller positive segments or continental 
areas left between these major segments might be expected to have 
triangular outlines, or at least, fewer angles than the major con- 
trolling segments. This hypothesis allows of a greater variety of 
shapes and it is easier to conceive that continents and sea areas 
conform roughly to these outlines. 

When smaller features, such as mountain chains, are considered, 
the linear distribution, more or less near and parallel to contacts 
of positive and negative elements, is obvious. As one notes the 
great extent and persistence of these linear elements and notes the 
synchronism of like deformation over large areas, he cannot but 
suspect that the major earth deformation as a whole may ulti- 
mately be reduced to simpler terms than a casual inspection of the 
irregularities of the surface might suggest. 

1 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. 1, pp. 521-522. 



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MAJOR STRUCTURES 143 

ACTUAL AND APPARENT UPLIFTS 

Actual uplift of the earth's crust may come about through the 
rigidity of the crust, allowing tangential thrust to be transformed 
into uplift, or through increase in volume. It seems to be demon- 
strated that the crust is rigid only on a small scale (see p. 145), 
and that actual uplift, due to rigidity, can affect only a small area. 
Uplift due to increase in volume may be shown also to have very 
narrow limits. The larger uplifts are probably apparent, not 
actual, and may be caused by the lowering of sea levels brought 
by sinking of earth segments. In other words, the earth move- 
ments are dominantly centripetal, and of varying intensity, with 
the result that certain areas appear to rise. 



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ULTIMATE FORCES OF SECONDARY DEFORMATION 

OUTLINE OF PRINCIPAL THEORIES 

Secondary structures, both on a large and small scale, are essen- 
tially the result of failure of the earth's crust, and are indeed 
evidence of this failure. Depending somewhat on one's point of 
view with reference to the origin of the earth, the stresses causing 
this failure have been ascribed to the cooling of a thin shell around 
a liquid core; to the redistribution of temperatures in a solid 
earth, heat from the center moving to the outer portion faster 
than radiated from the outer portion into space; to erosion causing 
a disturbance of equilibrium between different segments, thereby 
releasing the potential energy available in differences of density in 
adjacent masses; and to other causes. A consideration of these 
forces involves a discussion of hypotheses of the origin of the earth, 
which it is not the purpose here to attempt. We are concerned 
primarily with the manner in which these forces are localized and 
directed, not with the ultimate sources of the stresses. 

Stripped of detail and modifying considerations there appear to 
be two main hypotheses to account for deformation of the earth's 
shell. 

First: The cooling and shrinking of the nucleus faster than the 
shell causes the shell to collapse. In collapsing, strong tangential 
thrusts are set up; the rocks become deformed primarily by these 
thrusts, and subordinately by local tensional stresses near the 
surface. Notwithstanding this failure as a whole, it is conceived 
that the rocks are sufficiently rigid to transmit thrusts for long 
distances, developing and maintaining by their rigidity not only 
mountain ranges, but geanticlines and geosynclines, plateaus, con- 
tinents, oceanic basins, and other large units of structure. This is 
the old, and present popular, conception of earth deformation. 

Second : Deformation based on quite a different principle is that 
which results from the disturbance of isostatic equilibrium between 
the segments of the earth which are of different density. So far 

144 



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ULTIMATE FORCES 145 

as the different parts of the earth are in isostatic equilibrium, the 
transfer of loads by erosion from light to heavy segments may so 
disturb this balance between heavy and light segments as to cause 
a compensating flow of rock material beneath the surface, resulting 
in rock deformation. This principle of deformation is indei>endent 
of that postulated in the preceding paragraph, but the two may lw 
combined in any ratio; one does not necessarily exclude the other. 
The first hypothesis emphasizes the strength of rocks, the 
second, weakness of rocks. The first hypothesis in its simpler 
features is sufficiently well known not to require further elucida- 
tion here. The second is discussed below. 

ISOSTASY 

SUPPORT OF HYPOTHESIS BY RECOGNITION OF WEAKNESS OF 

ROCKS 

In the past the strength and rigidity of rock masses was supposed 
to be sufficient to develop and maintain major elevations and de- 
pressions of the earth's surface. It was supposed that the shorten- 
ing of the earth's crust necessarily accounted for the lilting of great 
areas, possibly even of continental areas, on the arch principle. 
Gradually it came to be realized that this was demanding too much 
of the strength of rocks — that rocks in large masses on the scale of 
the earth are weak. Chamberlin 1 cites calculations to show that 
a dome, with the curvature of the earth, would support only m of 
its own weight. 

The recognition of the weakness of rocks favored the wider 
acceptance of the hypothesis of isostasy to explain the major in- 
equalities in the earth's surface, namely that the inequalities are 
due to differences in density of the rock masses low density of 
certain rocks, and hence greater specific volume, making them 
stand higher above the surface than rocks in adjacent areas with 
greater density and hence smaller specific volume. Continental 
areas as a whole, then, would be areas with rocks of low density 
compensated by higher elevations. The sea bottoms would be 
areas of high densities of rocks compensated by the depressions. 

The causes of the differences in density required by the isostatic 

1 Chamberlin, T. C., and Salisbury, R. D., Geology, Vol. 1, 1904, pp. 555-556. 



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146 STRUCTURAL GEOLOGY 

theory are not material to the discussion. We are concerned with 
proof of the existence of the differences in density. Parenthetically 
it may be remarked that Chamberlin believes that according to the 
planetesimal theory of the formation of the earth the effect of 
differential weathering and of vulcanism would tend continuously 
to arrange the densities in the growing earth in their present dis- 
tribution. 1 

DUTTON'S AND GILBERT'S OBSERVATIONS ON ISOSTASY 

Dutton 2 proposed this theory in connection with his study of 
western mountains. Gilbert, 3 analyzing and discussing the gravity 
determinations of Putnam of the Coast and Geodetic Survey, con- 
cluded "the measurements of gravity appear far more harmonious 
when the method of reduction postulates isostasy than when it 
postulates high rigidity. Nearly all the local peculiarities of 
gravity admit of simple and rational explanation on the theory 
that the continent as a whole is approximately isostatic, and that 
the interior plain is almost perfectly isostatic. ,, 

HAYFORD'S OBSERVATIONS ON ISOSTASY 

Many more observations of the Coast and Geodetic Survey 4 
under the immediate charge of Mr. John F. Hayford, have made it 
possible to state more definitely to what extent any large portion 
of the United States meets the requirements of isostasy. At some 
hundreds of stations in the United States the deflection of the 
plumb bob from the astronomic vertical was determined. With the 
aid of topographic maps, the lateral pull upon the plumb bob by 
topographic elevations was calculated, without, of course, assign- 
ing any deficiency of density to the elevated areas. The calculated 
deflection from the vertical, under the influence of the topography, 
was in each case found to be much larger than the actually observed 
deflection, though usually in the same direction. The obvious 

1 Chamberlin, T. C, and Salisbury, R. D., Geology, Vol. 2, 1906, pp. 106-110. 

2 Dutton, C. E., On some of the greater problems of physical geology: Bull. Phil. 
Soc. of Wash., Vol. 11, 1889, pp. 51-64. 

3 Gilbert, G. K., Notes on the gravity determinations reported by Mr. G. R. 
Putnam: Bull. Phil. Soc. of Wash., Vol. 13, 1895, p. 73. 

4 Hayford, John F., The figure of the earth and isostasy from measurements in 
the United States. Washington, 1909. Also Supplementary investigation in 1909 
of the figure of the earth and isostasy. Washington, 1910, and The effect of topog- 
raphy and isostatic compensation upon the intensity of gravity. Washington, 1912. 



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ISOSTASY 147 

inference was that there is a counteracting pull downward due to 
excess of density at that point; in other words, that there is excess 
of density in the topographic depressions corresponding to de- 
ficiencies in the elevations. The following quotation is from Hay- 
ford: 

"The logical conclusion from the study of the geoid contours for 
the United States, taken in connection with the fact already noted 
that the computed topographic deflections are much larger than 
the observed deflections of the vertical, is that some influence 
must be in operation which produces an incomplete counter- 
balancing of the deflections produced by the topography, leaving 
much smaller deflections in the same direction. . . ." 

"Both the general approximate studies for the whole world of 
the necessary effects of the known topography in producing de- 
flections of the vertical, and the detailed exact study made for the 
United States alone, by means of computed topographic deflec- 
tions and geoid contours, indicate that one must look to the dis- 
tribution of the subsurface densities for an explanation of the dis- 
crepancies between observed deflections of the vertical and the 
deflections which must inevitably be produced by the topography. 
Moreover, from the general considerations set forth in the preced- 
ing paragraphs, it seems that there must be some general law of 
distribution of subsurface densities which fixes a relation between 
subsurface densities and the surface elevations such as to bring 
about an incomplete balancing of deflections produced by topog- 
raphy on the one hand against deflections produced by variation 
in subsurface densities on the other hand. . 

The theory of isostasy postulates precisely such a relation be- 
tween subsurface densities and surface elevations. . . ." 

"Keeping this contrast in mind, the writer believes that the 
stress-differences in and about the United States have been so 
reduced by the isostatic compensation that they are less than one- 
twentieth as great as they would be if the continent were main- 
tained in its elevated position and the ocean floor maintained in 
its depressed position by the rigidity of the earth. . . ." 

"It is certain, from the results of this investigation, that the 
continent as a whole is closely compensated, and that areas as 
large as States are also closely compensated. It is the writer's 
belief that each area as large as one degree square is generally 



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148 STRUCTURAL GEOLOGY 

largely compensated. The writer predicts that future investiga- 
tions will show that the maximum horizontal extent which a 
topographic feature may have and still escape compensation is 
between 1 square mile and 1 square degree. This prediction is 
based, in part, upon a consideration of the mechanics of the prob- 
lem." * 

EARTH MOVEMENTS IN RELATION TO ISOSTASY 

When one keeps in mind the fact that erosion is continuously 
shifting the load, and that there is local evidence of the erosion of 
thousands of feet of sediments, it must be inferred, if there is the 
present high degree of isostatic adjustment postulated above, that 
the process of isostatic adjustment is a continuous one, accom- 
plished by deep-seated rock flow keeping pace with the transporta- 
tion of surface material. Movements thus initiated should cause 
other movements, principally near the contacts of the positive 
elements of low density with the negative elements of high density 
(see p. 141). Initial dip of sediments in these areas would still 
further localize deformation. 

ISOSTASY IN RELATION TO RIGIDITY OF ROCKS 

If the United States, as well as certain other parts of the world, 
is in a state of isostatic equilibrium to such a remarkable degree, 
it would follow that the major irregularities of the surface are not 
due to the rigidity of the rocks, but rather to their weakness. If 
their rigidity were sufficient to account for the irregularities, there 
would be no need of the theory of isostatic adjustment to explain 
them, and there would be great variations from such a state of 
equilibrium. Isostasy and rigidity are mutually exclusive on any 
large scale. If the rocks were adequately rigid, it would be im- 
possible for them to yield sufficiently to accomplish a delicate 
isostatic adjustment. But rigidity is effective to some extent, 
notwithstanding this tendency toward adjustment, for small 
units — to what extent is still an open question. 

Rigidity is sufficient to account for periodicity in major earth 
movements. During long periods of quiet the rocks seem to have 

1 Hayford, John F., The figure of the earth and isostasy from measurements in 
the United States: Coast & Geodetic Survey, Washington, 1909, pp. 65, 66, 166, and 
169. 



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ISOSTASY 149 

been sufficiently rigid to have allowed the enormous stresses to 
accumulate which found expression in mountain-making periods. 

DEPTH OF ISOSTATIC COMPENSATION 

The differences in density postulated by isostasy cannot be ex- 
pected to extend downward indefinitely; in fact, the theory was 
developed to accord with the then prevailing notion that beneath 
the solid shell was a liquid or a near-liquid substratum upon which 
the shell rested or floated. The depth through which the differences 
of density were supposed to extend has been called the depth of 
compensation. A plane at this depth would support equal weights 
of material above, regardless of their density; below this, the den- 
sity is supposed to be uniform. Postulating the existence of such 
a plane of complete compensation, Hayford assumed various 
arbitrary depths in order to find out which one corresponded most 
closely to the facts of the gravity observations. For each of the 
arbitrary depths calculated, three alternative distributions of 
density were assumed — 1st, uniform distribution of density to 
the depth of complete compensation; 2d, a gradually diminishing 
difference in density to this depth; 3d, a maximum difference in 
density at some intermediate point. Depending on distribution 
of density chosen, the depth of complete compensation was cal- 
culated to be between 60 and 150 miles. With a uniform distribu- 
tion of density a depth of compensation of 76 miles was found best 
to correspond with the plumb bob observations. The discrep- 
ancies are so slight that Hayford concludes that the area of the 
United States falls one-tenth short of complete isostatic adjust- 
ment. 

CRITICISM OF THEORY OF ISOSTASY 

Unquestionably the plumb bob deflections show the existence 
of some sort of isostatic compensation, the higher areas having 
lower density and the lower areas having higher density; but it is 
questionable whether the compensation is as complete as indicated 
by Hayford. Lewis * has called attention to the fact that the con- 
ception of the existence of a plane of complete compensation by 
Hayford is an assumption, as in fact is so stated by Hayford; that 
having found the depth of such an hypothetical plane which would 

1 Lewis, Harmon, The theory of isostasy: Jour. Geol., Vol. 19, 1911, pp. 603-626. 



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150 STRUCTURAL GEOLOGY 

most nearly satisfy the requirements of the inferences from ob- 
servation, it is not permissible in turn to use this hypothetical 
depth as a standard against which to measure variations from the 
requirements of the assumption of isostatic compensation at this 
depth. It is, in effect, reasoning in a circle. Lewis showed that 
similar close accord with the observations could be secured by as- 
suming partial compensation at less depths, or over-compensation 
at greater depths. In other words, while the facts clearly in- 
dicate some sort of compensation, they do not so clearly dis- 
criminate between complete compensation, under-compensation, 
and over-compensation. Rigidity of the rocks is known at the 
surface to play some part — it may be a considerable part — in 
preventing complete isostatic adjustment. In so far as it is im- 
portant, it favors the assumption of under-compensation or over- 
compensation, rather than complete compensation. 

Hayford's reply to this argument is that the actual detailed 
observations and computations yield results more nearly accordant 
with his assumption of complete compensation than with assump- 
tions of over-compensation or under-compensation. 1 

From the geological standpoint there are difficulties in the way 
of the complete acceptance of the theory of isostasy because of the 
fact that areas of uplift and depression or areas of erosion and 
deposition have not been continuously such during geological 
history; a given area is likely to be one of alternate uplift and de- 
pression and of alternate erosion and deposition. If uplift and de- 
pression are related to density, as assumed by the isostatic theory, 
these alternations of uplift and depression require alternations of 
states of density, which is not satisfactorily explained under the 
isostatic theory. 

There seems also to be objection on the ground that differences 
in density could not be maintained, especially in the zone of rock 
flowage, and therefore would not for long be a source of deforma- 
tion. If, on the other hand, the rocks are rigid enough to maintain 
these differences in density, and the loading of the denser segments 
by sedimentation is sufficient to start movement toward the lighter 
segments, the question naturally arises as to the reason for the 
absence of movement in the opposite direction before the erosion 

1 Hayford, John F., Isostasy, a rejoinder to the article by Harmon Lewis: Jour. 
Geol., Vol. 20, 1912, pp. 562-578. 



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ISOSTASY 151 

and deposition took place. If there was isostatic equilibrium in 
the first instance, then at some point above the plane of compensa- 
tion, whether it was complete or partial, stresses must have been 
acting from the lighter and higher segments toward the heavier 
and lower. According to the theory of isostasy rocks are rigid 
enough to prevent this actual movement; and yet it is argued that 
movement should occur when the situation is reversed and stresses 
of equal (or less?) magnitude are set up in an opposite direction by 
erosion of the lighter segments and deposition on the heavier ones. 

The fact of high areas being light and low areas being dense does 
not necessarily imply that the difference in density is the cause of 
the differences in elevation or deformation. This latter may be an 
incidental accompaniment or may be the result of deformation by 
thrust or gravity. Deformation of rocks under thrust or gravity 
stresses is localized in the weakest places. It may be, then, that 
the light areas are weaker than the heavy ones. They would tend, 
therefore, to be folded and crowded up. In one sense, then, the 
high areas may be high because they are light and weak; but this 
is quite a different conception of the nature and causes of deforma- 
tion from that postulated by isostasy. The facts cited to support 
isostasy are fully as well in accord with such an alternative hypoth- 
esis of deformation. 

Again, it is possible that light areas are light because they 
are high, and not high because they are light. The processes 
of katamorphism, which increase the volume and decrease the 
density of rocks, affect higher areas to a greater extent than lower 
water-covered areas. This is undoubtedly a real factor, but 
whether sufficiently important to explain any considerable part 
of the observed differences in density is not yet known. 

The inference from gravity observations that high areas are 
generally light, applies principally to broad areas of uplift and not 
to the minor units of structure. The highest peaks are determined 
essentially by their resistance to erosion and not alone by their 
density. In certain parts of central Brazil the highest peaks are 
hard hematite, with a specific gravity of 5, which happens to be 
the most resistant material in this region. These particular peaks 
would not be explained on the isostatic principle, but when taken 
in connection with the broad area of uplift of which they are a part, 
the principle might still hold. 



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152 STRUCTURAL GEOLOGY 

It may be concluded that a condition of isostasy exists, but to 
what degree is still a matter of doubt. The disturbance of this 
condition is a probable factor in the deformation of rocks, but 
there are other important and perhaps more important factors. 

CAUSES OF TENSION 

In the above discussion of major causes of the earth's deforma- 
tion nothing has been said about tension, for in fact the major de- 
formation of the earth has been by tangential compression, result- 
ing in mountain chains and overthrust faults, whereas tension 
structures have been usually regarded as local and subsidiary. 
In connection with the discussion of tension joints and tension 
faults on previous pages (see pp. 22, 39) local conditions causing 
tension have been cited. That tension is present on any large 
scale is not certain. 

Neither the so-called contractional theory of earth deformation 
or the theory of isostasy discussed above imply the existence of 
tension in our zone of observation as anything but subsidiary and 
consequent upon thrusts. Under the contractional theory tension 
is produced in the earth's shell when the circumferential shortening 
by cooling predominates over compression and thrust in the shell 
due to radial shortening. At the surface and to a depth of a few 
miles, the circumferential contraction by cooling is at a minimum, 
whereas thrust due to collapse of the shell is at a maximum. 
Deeper below the surface cooling is going on more rapidly and it is 
supposed that the circumferential shortening, involving tension, 
may predominate over a thrust, though at this depth rock flowage 
might prevent actual tensional openings. At some intermediate 
depth, called the level of "no strain," it has been presumed that 
the circumferential shortening just equalized the thrust due to 
collapse and there would be no lateral tension or compression. 
This theory therefore implies no general state of tension within our 
zone of observation. 

The deformation involved in the disturbance of isostasy like- 
wise does not imply tension except locally. 

CONCLUSION AS TO MAJOR CAUSES OF DEFORMATION 

We conclude that earth deformation is principally due to 
gravity, locally transformed into thrust, and causing a collapse 



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MAJOR CAUSES OF DEFORMATION 153 

and buckling of the earth's shell; that the known differences in 
density between higher and lower areas indicate some sort of an 
isostatic adjustment; that this isostatic adjustment may be an 
accompaniment or result of mechanical thrust, or that it may be 
an initial condition, the disturbance of which by erosion would 
cause deformation, independent of any majcr thrust due to the 
collapse of the earth's shell; that tension is local and subsidiary to 
thrust. 

This conclusion throws some emphasis on the competence of the 
earth to transmit thrusts and to cause and sustain large uplifts. 
It is believed that this is possible: (1st) because of the competence 
of the beds of the zone of rock fracture, and (2d) because of the 
actual squeezing up of the rock material from below in the zone 
of flow, this squeezing possibly affecting the lighter rather than 
the heavier material. There seems to be no reason why the crowd- 
ing together of material by rock flowage in a deep-seated zone 
should not account for major uplifts in which the surface buckling 
seems small, as in the Cascade Range. The great pressures in the 
zone of rock flowage may impart a high degree of rigidity to the 
mass capable of transmitting thrusts — in spite of the fact that the 
rock flows. 

Whatever the cause of deformation, it is apparent that the 
earth's shell is, as a whole, a weak or failing structure. The 
secondary structures which have been described are evidences of 
failure. Rigidity has not prevented failure except for the smallest 
units — it has postponed failure, and favored a certain periodicity 
to earth movements. 

LOCAL AND MINOR CAUSES OF DEFORMATION 

Weathering involves increase in volume of some rocks. This 
increase in volume sets up compressive strains sufficient for minor 
local deformation. Some minor folding has been attributed to 
this cause. 1 

The purely mechanical effects of heating and cooling at the sur- 
face are known to produce local deformation. (See pp. 22, 25.) 

Removal of a load by erosion from a rock under compressive 
strain (for whatever cause) may give sufficient relief to allow def- 

1 Campbell, D. F., Rock folds due to weathering: Jour. Geol., Vol. 14, 1906, 
pp. 718-721. 



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154 STRUCTURAL GEOLOGY 

ormation of the rock. It is not uncommon in quarry and other 
underground excavations for rocks to swell and buckle when the 
superimposed pressure is removed. 

Unconsolidated sediments in a drift may be deformed when 
crowded or overridden by a glacier. Overthrust folds may be 
thus developed. 

RELATION BETWEEN DEFORMATION AND VULCANISM 

In regions of igneous rocks evidences of rock deformation are 
likely to be unusually numerous and conspicuous. For instance, 
cleavage is sometimes well developed in rocks which have been 
intruded by a great batholith, as in the Black Hills area of South 
Dakota. Joints and faults are abundant in areas of volcanic 
activity as is shown in the maps of some of the western mining 
districts (see p. 43). It is frequently possible to infer that the 
faulting closely followed and perhaps accompanied the intrusion 
of the igneous rocks. Shattering of wall rocks near contacts with 
intrusives is a commonly observed phenomenon. Presumably 
mechanical pressures and temperature changes combine to produce 
this result. 

Not less obvious is the tendency for igneous rocks when in- 
truded to follow pre-existing joint and fault planes or to be de- 
flected in their course by folds. The association of vulcanism with 
mountains is well known. 

Earthquakes are both the cause and result of rock deformation. 
Some earthquakes are related to vulcanism both in time and place 
(see page 70). 

These various relations indicate a genetic relationship between 
secondary structures and igneous activity. A broader view of the 
situation is that both vulcanism and the development of secondary 
structures are closely related effects of great earth movements. 
It has been shown to be probable that deep in the zone of flowage 
rocks are at such temperatures that they would liquefy if the pres- 
sure upon them were not so great. A change of conditions result- 
ing from any great earth movement, whatever its cause, may 
tend to disturb the equilibrium between pressure and temperature 
and allow the rock to liquefy. Having then less density than the 
unliquefied rocks, it moves upward. 



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DEFORMATION AND VULCANISM 155 

From this point of view both vulcanism and secondary deforma- 
tion are the results of great readjustment in major segments of the 
earth's shell. Looked at on a smaller scale, vulcanism and de- 
formation are found to have mutually reacted, with the result 
that either may be in a causal relation to the other, as in the 
illustrations given above. 



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UNCONFORMITY 

Contiguous formations are said to be unconformable when there 
is evidence of an erosion interval of some magnitude between their 
periods of formation or evidence of cessation of deposition between 
them. In either case there is loss of part of the geological record. 
The term unconformity is sometimes used to indicate primarily 




Fig. 66. Horizontally bedded limestone, resting unconformably on vertical beds 
of Proterozoic quartzite. Box Canyon, near Ouray, Colo. After R. T. Cham- 
berlin. 

the physical discordance ; sometimes it is applied principally to the 
time interval implied by the discordance; it usually implies both. 
The evidences of unconformity cited below are both physical and 
organic. The secondary deformation of rocks with which this 
book is mainly concerned is only one of the factors to be considered 
in unconformity. Stratigraphy, physiography and paleontology 
are others, — in fact adequate understanding of the significance of 

156 



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UNCONFORMITY 



157 



unconformity involves the widest range of geological knowledge. 
The subject is treated here principally in its relation to structural 
geology, and not in the broader sense that it is required for a 
philosophical understanding of its significance. Involving, as it 
does, considerations other than structural, it has been left to the 
last chapter. 

IDENTIFICATION OF UNCONFORMITY 

Physical evidences of unconformity are: 

(1) Evidence of erosion, even without intervening deformation 
between formations. 

(2) Difference in Metamorphism: — Stratigraphically lower rocks 
may have suffered so much more metamorphism than overlying 




Fig. 67. Ideal sketch to illustrate unconformities. After Spurr. A. Earlier line 
of conformity; B. Later line. 

beds of similar lithology as to indicate the probability of a time 
interval between them. Original differences in lithology also 
influence the nature and extent of metamorphism. This fact 
should not be overlooked. 

(3) Difference in Deformation: — Stratigraphically underlying 
rocks may be folded or cracked or may be schistose as result of 
flowage, while these features may be less conspicuous or lacking in 
upper beds of similar kinds, indicating a time interval between 
their periods of formation. This criterion must be carefully used, 
for the differences in deformation may be due simply to varying 
competence of the different beds. 

(4) Difference in Number of Igneous Intrusions: — Stratigraphi- 
cally underlying beds may be intruded by igneous rocks, which 
have not intruded the upper beds. This may not in itself be 
evidence of unconformity, but may confirm other evidences of the 



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158 STRUCTURAL GEOLOGY 

existence of an erosion interval between lower and upper beds. If 
the igneous rock in the lower bed is a plutonic rock and appears on 
the contact erosion plane, it is evidence that the erosion interval 
has been of sufficient duration to allow of the removal of a great 
thickness of rock. 

(5) Basal conglomerate in the upper beds, carrying fragments 
from the rocks beneath the contact plane. If this conglomerate 
contains a variety of fragments derived from a considerable area, 
it is more significant of a time interval perhaps than a conglomerate 
made up of fragments entirely like the immediately underlying 
rock. However, if the underlying rocks are homogeneous over 
great areas, the overlying basal conglomerates may show a marked 
homogeneity of fragments. Intraformational conglomerates are 
sometimes formed by exceptional storms or other causes, in the 
course of a continuous deposition of sediments. Such conglomer- 
ates mark no erosion interval of magnitude and have little signif- 
icance with reference to unconformity. 

While a basal conglomerate indicates unconformity, the absence 
of such a conglomerate does not disprove unconformity, for 
students of sedimentation now find many conditions under which 
sediments may be deposited unconformably on an older surface 
without intervening conglomerates. The base of the Paleozoic 
in the Mississippi Valley as a whole is remarkably free from basal 
conglomerates except near monadnocks on the old pre-Cambrian 
peneplain. The Niagara limestone resting on the pre-Cambrian 
rocks of the Cobalt district of Ontario furnishes a fine example of 
unconformable contact without basal conglomerate. 

(6) Field relations and areal distribution of rocks may indicate 
an unconf ormity even where actual contacts or other evidences are 
lacking. For instance, a continuous bed of quartzite lying along- 
side of a heterogeneous group of rocks with irregular distribution 
would in itself suggest unconformity between these rocks and the 
quartzite. This criterion of field relations is of the utmost practical 
importance. It is frequently possible from a preliminary study of 
maps showing areal distribution of lithologic types to infer possible 
unconformities, and if so, to direct further field work with much 
greater effectiveness than would otherwise be possible. 

(7) Difference in lithology; as, for instance, where a sedimentary 
rock rests upon an igneous rock without intrusive relations. 



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UNCONFORMITY 159 

(8) There may be an irregular erosion surface separating parallel 
strata. Differences in lithology on the two sides of the contact or 
fossil evidence may aid in determining this surface. 

(9) Hiatus in the fossil record between successive beds. 

(10) Absence of rocks between successive beds known elsewhere 
to have been deposited in this relation. 

Commonly the greater number of these criteria can be used in 
working out unconformity. One line of evidence can usually be 
substantiated by others. 

INTERPRETATION OF UNCONFORMITY 

Unconformity represents a lost interval not otherwise recorded 
at that place. This lost interval may involve (a) a cessation of 
deposition, usually involving emergence, and often accompanied 
by deformation of the rocks; (b) denudation, usually by subaerial 
processes; (c) resumption of deposition, usually following sub- 
mergence, but often by terrestrial processes. 1 

The appraisement of the value of an unconformity requires much 
care. The terms "great" and "slight" frequently applied to 
unconformity, express the value very crudely. By great uncon- 
formity may be meant one in which there is a prominent discord- 
ance of structure, or one indicating the absence of great thicknesses 
of strata, or a long lapse of time, or any combination of these 
features. Usually it is intended to imply that the discordance is 
pronounced and that there is a great loss of record. It is desirable, 
wherever possible, that these factors be discriminated, even though 
their quantitative value cannot be determined. 

The study of unconformities broadly as continental features is of 
significance to structural geology as indicating the major warpings 
and oscillations of the continent with reference to the sea. If 
oceanic basins have been permanent during geological time, it may 
be supposed that there are no unconformities indicated by strata 
there deposited. However meager, the record may be one of con- 
tinuous deposition. The continents, however, from the beginning 
of the geological record have always in some part stood above 
water, have in some part been undergoing erosion, and therefore 

1 Blackwelder, Eliot, The valuation of unconformities: Jour. Geol., Vol. 17, 
1909, p. 290. 



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160 



STRUCTURAL GEOLOGY 



fall short of a complete record of deposition. By migrating from 
place to place during continental movements, animals might 
conceivably have lived continuously on the erosion surfaces which 
marked unconformities in the geologic record. Thus it appears 





TEXAS 
REGION 


CENTRAL 

INTERIOR 


LAKE 
REGION 


LABRADOR! 


Quaternary 










Tertiary 










Cretaceous 














Jurassic 










Triassic 










Permian 








Perm sy 1 van} an 








Misstssippian 










Devonian 










Silurioa 










Ordovfcian 




Cambrian 














Tre- Cambrian 











Fig. 68. Diagram of an unconformity with lateral extensions and restrictions. 
After Blackwelder. The extent and duration of the principal periods and 
areas of sedimentation, with their corresponding rock systems, are shown in 
solid black. The white, on the other hand, denotes the time and extent of 
erosional conditions and corresponding unconformities. 

that in one sense unconformities are continuous physically and 
chronologically; but they shift back and forth across the continents 
with successive oscillations and inundations. It is equally true 
that any localized unconformity is represented somewhere else by 
a continuous record of deposition. As Blackwelder states it: "The 
entire geologic record, then, is not to be conceived of as a pile of 



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UNCONFORMITY 101 

strata, but as a dovetailed column of wedges, the unconformities 
and rock systems being combined in varying proportions. The 
former predominate in some places and periods, while the latter 
prevail in others. ,, * 

SUGGESTIONS FOR LABORATORY STUDY OF UNCON 

FORMITY 

1. What different kinds of contacts, and, therefore, different relations 
between rock masses, can be found on the Three Forks and Livingston, 
Montana, geologic maps? (geologic folios U. S. Geol. Survey, Nos. 24 
and 1). 

2. What different kinds of field evidence for unconformity can be 
found on the following geologic maps: Three Forks, Montana (geologic 
folio No. 24, U. S. Geol. Survey), Holyoke. Mass. (geologic folio No. 50, 
U. S. Geol. Survey), Milwaukee, Wis. (geologic folio No. 140, U. S. Geol. 
Survey), Mount Stuart, Wash, (geologic folio No. 106, U. S. Geol. Survey), 
Hartville, Wyo. (geologic folio No. 91, U. S. Geol. Survey), maps of the 
Mesabi, Gogebic and Marquette districts of Lake Superior (Mon. 52, U. 
S. Geol. Survey). 

3. The historical significance of various unconformities: 

a. On the Hartville, Wyo., geologic map (geologic folio No. 91, U. S. 
Geol. Survey) determine the stratigraphic hiatus in terms of forma- 
tions, at several different points in the district. 

b. The same for relative degree of discordance between the beds. 

c. By studying the geologic maps in the following U. S. Geological 
Survey folios determine as closely as possible the time value of the 
unconformity at the base of the coastal plain in eastern United 
States: Trenton, N. J. (folio No. 167), Washington, D. C. (folio 
No. 70), Mercersburg-Chambersburg, Pa. (folio No. 170), Rome, 
Ga. (folio No. 78), and Knoxville, Tenn. (folio No. 16). 

1 Op. Cit., p. 299. 



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INDEX 



Adams, F. D., 4, 5, 6, 7, 9, 10, 11. 88, 
89, 90, 92, 100, 102 

Adams, L. H., 76 

Adirondack graphite deposits, sedi- 
mentary origin of, 99 

Alabama. See Gadsden 

Alaska, earthquakes at Yakutat Bay, 
70; fault scarp, 58; folded schist, 
112; fracture cleavage, jointing and 
flow cleavage in graywacke and 
slate, 65 

Alps, "decken" structure and folding, 
106, 117, 138; faults, 60, 137; 
topography influenced by folding 
or faulting, 140 

Anthracite - Crested Butte folio, 
faults, 60 

Anorthosite, Morin, granulation, 88, 
90 

Appalachians, northern, depth of 
folds, 125 

Appalachians, southern, absence of 
fault scarps, 59; compression 
joints, 23; contrast between frac- 
ture and flow, 13; determination 
of fault displacements, 38; dis- 
tributive and thrust faults, 46, 48, 
137; folded thrust fault planes, 51, 
54; folding, 111, 124, 137, 138; 
intimate relation of thrust faults 
and overthrust folds, 51; sliced 
feldspars in micaceous and chloritic 
schist, 83 ; topography influenced by 
folding or faulting, 140. See also 
Piedmont Plateau 

Arizona, faults, 44, 57. See also 
Clifton, Globe, Grand Canyon 



Atwood, W. W., 60, 133 
Autoclastics, 64 

Baltimore gneisses of Piedmont Pla- 
teau, 98 

Bannock overthrust in southeastern 
Idaho, 51, 60 

Baraboo district, Wisconsin, folding 
of slate, 129; fracture cleavage and 
jointing in quartzite, 23, 24, 26, 31, 
64, 121; vertical section of Illinois 
Mine, 129 

Barlow, A. E., 100 

Bascom, F., 98 

Basin Ranges, faults in, 43, 44, 56, 57, 
58, 137; laboratory study of moun- 
tains, 140; tension joints in, 22. 
See also Utah, Nevada, Arizona 

Basins, ocean, 141 

Bastin, E. S., 99, 100, 101 

Becker, G. F., 15, 29, 30, 44, 45, 77, 
85 

Bisbee district, Arizona, faults, 53, 
60 

Black Hills, South Dakota, chloritoid 
crystal in micaceous and quartzose 
schist, 91; cleavage, 63, 154; photo- 
micrograph of slate, 63 

Blackwelder, Eliot, iii, 159, 160 

Box Canyon, Colorado, unconform- 
ity, 156 

Brazil, determination of rocks by 
washing, 99; peaks of hematite, 151 

Breccias, 64 

Bristol folio, Virginia, folding, 134 

British Columbia, folding, 127 

Buckley, E. R., 17 



163 



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1G4 



INDEX 



Buffalo Mountain, Tennessee, theo- 
retical wet ion, 52 

Bullfrog district, Nevada, fault dis- 
placements, 39; extension by fault- 
ing, 56; hinge faults, 42 

Butte district, Montana, determina- 
tion of fault displacements, 39 

Cadell, H. M., 49, 51 

California. See Mother Lode, Santa 
Cruz, Yosemite Valley 

California earthquake, faults, 44, 50, 
57,59 

Campbell, D. F., 153 

Carolina gneiss of Piedmont Plateau, 
98 

Cascade Mountains, Washington, 
method of determining depth af- 
fected by folds, 125, 127; cause of 
uplift, 153 

Chamberlin, R. T., 125, 126, 127, 156 

Chamberlin, T. C, 36, 40, 44, 53, 
54, 121, 125, 127, 139, 142, 145, 
146 

Cleavage crossing bedding, 94, 95. 
See Flow Cleavage, Fracture Cleav- 
age 

Clifton district, Arizona, faults, 42, 
53 

Cloud Peak-Fort McKinney folio, 
folding, 134 

Clough, C. T., 49 

Cobalt district, Ontario, fractures in 
gabbro, 22; identification of con- 
glomerate, 66; unconformity, 158 

Coker, E. G., 92 

Colorado. See Anthracite-Crested 
Butte, Box Canyon, Georgetown, 
Silverton, Spanish Peaks 

Compression fractures, 16, 21 

Compression joints, 23 

Conglomerate, identification of, 66 

Connecticut. See Pomperaug Valley 

Continents, 141 

Cranberry folio, North Carolina, 
laboratory study of mountains, 
140 



Crosby, W. O., 67 
Crystalloblastic structure, 77 
Cushing, H. P., 36 
Cuyuna district, Minnesota, observa- 
tions on folding, 113 

Dale, T. Nelson, 26, 27, 31, 64 

Dalles of Wisconsin, drainage con- 
trolled by joints, 30; false bedding 
in sandstone, 133 

Daly, R. A., 127 

Dana, James D., 105 

Daubree, A., 15, 29 

Day, Arthur L., 29, 77, 85 

Decken structure, 117 

Deformation, ultimate forces of, 145 

Deformation and vulcanism, 154 

Derby, O. A., 99 

Distributive fault, 48 

Ducktown, Tennessee, sedimentary 
origin of gneisses, 98 

Dutton, C. E., 146 

Earthquakes, 67; and glaciers, 67; 
and magnetic disturbances, 70; 
and rock density, 70; and vulcan- 
ism, 70; as cause and effect of rock 
fractures, 67; location of, 73; pre- 
diction of, 74; seismograph, 71; 
waves, 72; waves in relation to 
earth's interior, 72; zones, 71 

Emmons, W. H., 39, 42, 56, 98 

Evans, John W., 36 

Faults, 31; correlation of, 53; dis- 
placements, apparent and real, 
36, 50; distributive thrust, 48; 
evidence of, 56; folded, 51; grading 
into folds or flowage, 51; hinge, 50; 
laboratory study, 60; nomencla- 
ture of, 32; normal, 39; normal, 
associated with folds, 43; normal, 
associated with igneous rocks, 42; 
normal, in intersecting systems, 44; 
normal, in unfolded sediments, 43; 
number of reverse and normal, 54; 
pivotal, 50; relative shortening 




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and elongation of the earth's crust 
by, 55; reverse or thrust, 46; sur- 
face expression of, 57. See also 
Fractures 
Fissility. See Fracture Cleavage 
Flow and fracture, conditions, 4; 
distribution, 2; kinds, 2; surface 
expression of zones, 3, 12; volume 
changes, 11 
Flow cleavage, 76; and folds, 119, 

129; and pressure, 83 
Flowage. See Rock Flowage 
Folds, 104; definitions, 104; depth, 
124; differential movement on 
limbs, 114; elements, 104; field 
observations, 127; fracture and 
flow contrasted, 108; laboratory 
study, 134; minor drag, 114, 128; 
relation to cleavage, 119, 128; re- 
lation to joints, 121; relation to 
faults, 43, 51 ; strike and dip obser- 
vations, 127 
Forces of secondary deformation, 144 
Fractures, 14, 19; tension, 14, 20. 

See also Joints, Faults 
Fracture cleavage, 61; compression, 
16, 21; in relation to earthquakes, 
67 
Fracture and flow. See Flow and 

Fracture 
Front Range, Colorado, folding, 138 
Front Range, Montana, thrust faults, 

53 
Futterer, Karl, 82 

Gadsden folio, Alabama, evidences of 
fracture and flow, 13 

Garrey, G. H., 39, 42, 56, 98 

Geanticlines, 141 

Geikie, Archibald, 49 

Georgetown area, Colorado, sedi- 
mentary origin of gneiss, 98 

Georgia. See Rome 

Geosynclines, 141 

Gilbert, G. K., 3, 44, 50, 57, 58, 59, 
65, 75, 146 

Glaciers and earthquakes, 67 



Globe district, Arizona, correlation of 
faults in, 53 

Gneiss, criteria for origin, 87, 97 

Gneissic structure, 87 

Gogebic district, Michigan, evidences 
of fracture and flow, 13; uncon- 
formity, 161 

Goldfield district, Nevada, hinge 
faults in, 42 

Grand Canyon, drainage controlled 
by joints, 30, 31 

Granulation in rock flowage, 82 

Grubenmann, U., 77 

Gunn, W., 49 

Hallock, William, 7, 9 

Harder, E. C, 31 

Harrisburg, Pennsylvania, section 
from, to Tyrone, 125, 126 

Hartville folio, Wyoming, uncon- 
formity, 161 

Hayford, John F., 146, 147, 148, 149, 
150 

Heim, Albert, 3, 106, 117, 118, 140 

Henry Mountains, existence of zones 
of fracture and flow, 3 

Highlands of Scotland, faults, 39, 49, 
51, 60, 137, 140 

Hinge fault, 50 

Hinxman, L. W., 49 

Hobbs, W. H., 44 

Holyoke folio, Massachusetts, un- 
conformity, 161 

Hoosac, Massachusetts, micaceous 
and quartzose schist, 80, 81 

Home, John, 49 

Hoskins, L. M., 9, 10, 16, 86 

Hotchkiss, W. O., 116 

Hurricane fault scarp, 56, 57, 58 

Idaho, Bannock overthrust in south- 
eastern, 51, 60 

Idiomorphic texture in rock flowage, 
90 

Illinois Mine, Baraboo district, Wis- 
consin, vertical section of, 129 

Imbricate structure, 49 



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INDEX 



Indian Territory. See Tahlequah 

Iron Springs district, Utah, hinge 
faults in, 50; tension joints in, 
23 

Isostasy, 145; criticism of, 149; 
Dutton's and Gilbert's observa- 
tions on, 146; Hayford's observa- 
tions on, 146; in relation to earth 
movements, 148; in relation to 
rigidity of rocks, 148 

Isostatic compensation, depth of, 149 

Jaggar, T. A., 36 

Johnston, John, 76 

Joints, 21, 25; and folds, 121; com- 
pression, 23, 28; in intersecting 
systems, 44; laboratory work on, 
31; of unknown origin, 28; surface 
expression of, 30; tension, 22, 26; 
widened by growing crystals, 29. 
See also Fractures 

Kaibab fault, Utah, 51 
Keith, Arthur, 51, 52, 96, 98 
Kick, Friedrich, 4 
Kingston earthquake, 72 
Knoxville folio, Tennessee, uncon- 
formity, 161 

Laboratory study of faults, 60; 
folds, 134; joints, 31; mountains, 
140; unconformity, 161 

Lake Superior Region, fracture and 
flow contrasted, 13; gneisses of, 
102; identification of tuffs, 66; 
identification of schists, 103 

Laney, F. B., 98 

Laurentian area north of Montreal, 
leaf gneiss from, 89 

Lawson, A. C, 60 

Leaf gneiss north of Montreal, 
89 

Lehmann, Johann, 102 

Leith, C. K., 66, 76 

Lewis and Livingston Ranges, labora- 
tory study of mountains, 140; 
thrust faults, 53 



Lewis, Harmon, 149, 150 

Limestone Cove, Tennessee, theo- 
retical section, 52 

Lindgren, Waldemar, 42 

Little Belt Mountains folio, Mon- 
tana, laboratory study of moun- 
tains, 140 

Little Falls, Minnesota, slaty cleav- 
age crossing bedding, 94 

Livingston folio, Montana, labora- 
tory study of mountains, 140; un- 
conformity, 161 

Livingston Range. See Lewis 

Lugeon, M., 117 

Magnetic disturbances and earth- 
quakes, 70 

Maine, granites, 31 

Mansfield, G. R., 51, 60 

Marquette district, Michigan, cor- 
relation of structures, 113, 115; 
evidences of fracture and flow, 13; 
unconformity, 161 

Martin, Lawrence, 58, 70 

Massachusetts, granites, 31. See 
also Holyoke, Hoosac 

Mathews, E. B., 98 

Maynardville, folio, Tennessee, fold- 
ing, 134 

Mead, W. J., iii, 119 

Mediterranean earthquake zone, 71 

Menominee district, Michigan, fold- 
ing, 117, 134; origin of green 
schists, 101 

Mercersburg-Chambersburg folio, 
Pennsylvania, unconformity, 161 

Mesabi district, unconformity, 1C1 

Messina earthquake, 71 

Michigan. See Gogebic, Lake Supe- 
rior, Marquette, Menominee 

Milch, L., 77 

Milne, John, 69, 72, 73 

Milwaukee folio, Wisconsin, uncon- 
formity, 161 

Minnesota. See Cuyuna, Lake Su- 
perior, Little Falls, Mesabi, St. 
Louis, Vermilion 



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Mississippi Valley, basal conglomer- 
ate, 158 

Montana. See Front Range, Lewis 
and Livingston Ranges, Little 
Belt Mountains, Three Forks 

Monterey folio, Virginia, folding, 134; 
laboratory study of mountains, 140 

Montessus de Ballore, F. de, 71 

Montreal, leaf gneiss, 89 

Morin anorthosite, granulation of, 88 

Morristown folio, Tennessee, faults, 
60; folds, 134 

Mother Lode district, California, 
widening of joints, 29 

Mount Mitchell folio, North Caro- 
lina, folding, 134 

Mount Stuart folio, Washington, un- 
conformity, 161 

Mountains, 136; laboratory study, 
140; localization of, 138; types of, 
136 



Pennsylvania, method of determin- 
ing depth affected by folds, 125. 
See also Mercersburg, Piedmont 
Plateau, Tyrone 

Pfaff, F., 7, 9 

Piedmont Plateau, cleavage, 120; 
folding, 111; fracture and flow con- 
trasted, 13; sedimentary origin of 
gneisses, 98. See also Appalachi- 
ans, southern 

Pisgah folio, North Carolina, evi- 
dences of fracture and flow, 13 

Pivotal fault, 50 

Plateaus, 141 

Pomperaug Valley, normal faults in 
intersecting systems, 44 

Porphyritic texture in rock flowage, 
90 

Positive elements, 141 

Protoclastic structure, 87 

Putnam, G. R., 146 



Negative elements, 141 

Nevada, faults in, 44, 57. See also 
Bullfrog, Goldfield, Great Basin, 
Tonopah 

New Hampshire, granites, 31 

New Jersey. See Trenton 

New Mexico. See Watrous 

Nicolson, J. T., 92 

Normal faults, 39. See also Faults, 
normal 

North Carolina. See Cranberry, 
Mt. Mitchell, Pisgah, Roan Moun- 
tain 

Ocean basins, 141 

Oelrichs folio, South Dakota, folding, 

134 
Ontario. See Cobalt 
Oregon. See Cascade 
Ozarks, topography influenced by 

folding or faulting, 140. See also 

Tahlequah 

Pacific earthquake zone, 71 
Peach, B. N., 49 



Ransome, F. L., 39, 41, 42, 56 

Reverse faults. See Faults 

Rhode Island, granites, 31 

Richards, R. W., 51, 60 

Ringgold, folioi Georgia, folding, 134 

Ripple marks, 131 

Roan Mountain folio, North Caro- 
lina, evidences of fracture and 
flow, 13; faults, 48, 52, 60; folded 
thrust fault planes, 51 

Rock density and earthquakes, 70 

Rock flowage, 76; evidence for the 
existence of zone, 3; grading into 
faulting, 51; obliteration of tex- 
tures, 93; without cleavage, 92 

Rome folio, Georgia, laboratory 
study of mountains, 140; uncon- 
formity, 161 

Rotation in rock flowage, 82 

Salisbury, R. D., 54, 60, 133, 139, 142, 
145, 146 

Santa Cruz folio, California, relations 
of valleys and lakes to fault dis- 
placements, 60 



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INDEX 



Saxony area, development of gneisses 
from granites, 102 

Scandinavian Highlands, faulting, 
137 

Schardt, Hans, 117 

Schists, identification, 97 

Schuppen structure, 48, 49 

Scotland. See Highlands 

Sierra Nevada Mountains, faults, 44, 
137 

Silverton folio, Colorado, faults, 
60 

Simplon Tunnel, rock flowage, 1 

Slicing in rock flowage, 83 

South Dakota. See Black Hills, 
Oelrichs 

Southern Appalachians. See Appala- 
chians 

Spanish Peaks folio, Colorado, labo- 
ratory study of mountains, 140 

Spurr, J. E., 36, 42, 43, 58, 98, 140, 
157 

Steidtmann, E., 26, 31 

Steinmann, G., 140 

St. Louis slates, cleavage crossing 
bedding, 95 

Suess, Edward, 139 

Sundance folio, Wyoming, laboratory 
study of mountains, 140; folding 
shown, 134 

Tahlequah folio, Indian Territory, 
laboratory study of mountains, 
140 

Tarr, R. S., 70 

Teall, J. J. H., 49 

Tennessee. See Bristol, Buffalo 
Mountain, Knoxville, Limestone 
Cove, Maynardville, Morristown, 
Ringgold, Walland 

Tension, causes of, 152; fractures, 14, 
20; joints, 22. See also Joints 

Terlingua, Texas, topographic map, 
faulting, 59 

Three Forks folio, Montana, folding, 
134; laboratory study of moun- 
tains, 140; unconformity, 161 



Thrust faults. See Faults 

Tolman, C. F., 36 

Tonopah district, Nevada, faults, 22, 
42, 43, 53 

Trenton folio, New Jersey, uncon- 
formity, 161 

Trueman, J. D., 78, 89, 100 

Tyrone, Pennsylvania, section, 125, 
126 

Unconformity, 156; identification, 
157; interpretation, 159; labora- 
tory study, 161 
Uplifts, actual and apparent, 143 
Utah, faults, 44, 57. See also Hurri- 
cane, Iron Springs, Kaibab 

Van Hise, C. R., iii, 3, 9, 10, 22, 
28, 53, 61, 66, 105, 107, 110, 121, 
131 

Vermilion district, Minnesota, fold- 
ing, 117, 128; pseudo-conglomer- 
ates, 66 

Vermont, granites of, 26, 31 

Virginia. See Monterey 

Vulcanism and deformation, 154; and 
earthquakes, 70 

Walland, Tennessee, cleavage and 

bedding, 96 
Wasatch Mountains, Hurricane fauft 

scarp, 56, 57, 58; normal faults, 4^, 

137 
Washington, D. C, identification of 

gneiss, 98; unconformity, 161 
Washington State. See Cascade 

Range, Mt. Stuart 
Watrous, New Mexico, joints, 31 
Weed, W. H., 39 
Weidman, Samuel, 129 
Wenatchee-Chelan district. See Cas- 
cade Range 
Williams, G. H., 99, 101 
Willis, Bailey, 33, 36, 46, 47, 53, 60, 

104, 111, 112, 122, 123, 124, 125, 

127, 135, 141 



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INDEX 169 

Wisconsin, southwestern joint sys- Yakutat Bay, Alaska, earthquakes at, 

tem, 31. See also Baraboo, Dalles, 70 

Milwaukee Yosemite Valley, drainage controlled 

Wright, F. E., 84 by joints, 30, 31 
Wyoming. See Cloud Peak, Hart- 

ville, Sundance Zone of flow contrasted with zone of 

fracture, 108 



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